Switch to PyData Sphinx theme

After merging various parts of Software Heritage documentation in a
single Sphinx instance, the navigation using the Read The Docs theme
became seriously cumbersome.

We switch to PyData Sphinx theme: a clean, Bootstrap-based Sphinx theme
by and for the PyData community. For details, please refer to:
https://pydata-sphinx-theme.readthedocs.io/

One major upside is that by default supports three level of navigation:

- top-level parts in the header bar (for us, they would refer to
  `devel`, `sysadm` and `user`)
- a left sidebar for sections in the current part,
- a right sidebar for the contents of the current page.

This maps really well to our hierarchy.

Some changes made to better navigate the various parts of our
navigation:

- Rename “Development Documentation” to just “Development”.
- Rename “Sysadmin Documentation” to just “Infrastructure”.
- Rename “User Documentation” to just “Usage”.
- Promote the API reference from Development to the top-level.

Some other improvements to better benefit the new theme:

- Replace the vertical logo with the horizontal one to better fit the
  header.
- Add links to GitLab, PyPI and status.s.o to the header.
- Remove the unused Alegreya font.

docs/_static/custom.css

-/* Alegreya font setup */
-
-/* alegreya-400normal - latin */
-@font-face {
-  font-family: 'Alegreya';
-  font-style: normal;
-  font-display: swap;
-  font-weight: 400;
-  src:
-    local('Alegreya Regular '),
-    local('Alegreya-Regular'),
-    url(fonts/alegreya-latin-400.woff2) format('woff2'),
-    url(fonts/alegreya-latin-400.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-400italic - latin */
-@font-face {
-  font-family: 'Alegreya';
-  font-style: italic;
-  font-display: swap;
-  font-weight: 400;
-  src:
-    local('Alegreya Regular italic'),
-    local('Alegreya-Regularitalic'),
-    url(fonts/alegreya-latin-400italic.woff2) format('woff2'),
-    url(fonts/alegreya-latin-400italic.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-500normal - latin */
-@font-face {
-  font-family: 'Alegreya';
-  font-style: normal;
-  font-display: swap;
-  font-weight: 500;
-  src:
-    local('Alegreya Medium '),
-    local('Alegreya-Medium'),
-    url(fonts/alegreya-latin-500.woff2) format('woff2'),
-    url(fonts/alegreya-latin-500.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-500italic - latin */
-@font-face {
-  font-family: 'Alegreya';
-  font-style: italic;
-  font-display: swap;
-  font-weight: 500;
-  src:
-    local('Alegreya Medium italic'),
-    local('Alegreya-Mediumitalic'),
-    url(fonts/alegreya-latin-500italic.woff2) format('woff2'),
-    url(fonts/alegreya-latin-500italic.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-700normal - latin */
-@font-face {
-  font-family: 'Alegreya';
-  font-style: normal;
-  font-display: swap;
-  font-weight: 700;
-  src:
-    local('Alegreya Bold '),
-    local('Alegreya-Bold'),
-    url(fonts/alegreya-latin-700.woff2) format('woff2'),
-    url(fonts/alegreya-latin-700.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-700italic - latin */
-@font-face {
-  font-family: 'Alegreya';
-  font-style: italic;
-  font-display: swap;
-  font-weight: 700;
-  src:
-    local('Alegreya Bold italic'),
-    local('Alegreya-Bolditalic'),
-    url(fonts/alegreya-latin-700italic.woff2) format('woff2'),
-    url(fonts/alegreya-latin-700italic.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-800normal - latin */
-@font-face {
-  font-family: 'Alegreya';
-  font-style: normal;
-  font-display: swap;
-  font-weight: 800;
-  src:
-    local('Alegreya ExtraBold '),
-    local('Alegreya-ExtraBold'),
-    url(fonts/alegreya-latin-800.woff2) format('woff2'),
-    url(fonts/alegreya-latin-800.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-800italic - latin */
-@font-face {
-  font-family: 'Alegreya';
-  font-style: italic;
-  font-display: swap;
-  font-weight: 800;
-  src:
-    local('Alegreya ExtraBold italic'),
-    local('Alegreya-ExtraBolditalic'),
-    url(fonts/alegreya-latin-800italic.woff2) format('woff2'),
-    url(fonts/alegreya-latin-800italic.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-900normal - latin */
-@font-face {
-  font-family: 'Alegreya';
-  font-style: normal;
-  font-display: swap;
-  font-weight: 900;
-  src:
-    local('Alegreya Black '),
-    local('Alegreya-Black'),
-    url(fonts/alegreya-latin-900.woff2) format('woff2'),
-    url(fonts/alegreya-latin-900.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-900italic - latin */
-@font-face {
-  font-family: 'Alegreya';
-  font-style: italic;
-  font-display: swap;
-  font-weight: 900;
-  src:
-    local('Alegreya Black italic'),
-    local('Alegreya-Blackitalic'),
-    url(fonts/alegreya-latin-900italic.woff2) format('woff2'),
-    url(fonts/alegreya-latin-900italic.woff) format('woff'); /* Modern Browsers */
-}
-
-
-/* alegreya-sans-100normal - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: normal;
-  font-display: swap;
-  font-weight: 100;
-  src:
-    local('Alegreya Sans Thin '),
-    local('Alegreya Sans-Thin'),
-    url(fonts/alegreya-sans-latin-100.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-100.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-100italic - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: italic;
-  font-display: swap;
-  font-weight: 100;
-  src:
-    local('Alegreya Sans Thin italic'),
-    local('Alegreya Sans-Thinitalic'),
-    url(fonts/alegreya-sans-latin-100italic.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-100italic.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-300normal - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: normal;
-  font-display: swap;
-  font-weight: 300;
-  src:
-    local('Alegreya Sans Light '),
-    local('Alegreya Sans-Light'),
-    url(fonts/alegreya-sans-latin-300.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-300.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-300italic - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: italic;
-  font-display: swap;
-  font-weight: 300;
-  src:
-    local('Alegreya Sans Light italic'),
-    local('Alegreya Sans-Lightitalic'),
-    url(fonts/alegreya-sans-latin-300italic.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-300italic.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-400normal - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: normal;
-  font-display: swap;
-  font-weight: 400;
-  src:
-    local('Alegreya Sans Regular '),
-    local('Alegreya Sans-Regular'),
-    url(fonts/alegreya-sans-latin-400.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-400.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-400italic - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: italic;
-  font-display: swap;
-  font-weight: 400;
-  src:
-    local('Alegreya Sans Regular italic'),
-    local('Alegreya Sans-Regularitalic'),
-    url(fonts/alegreya-sans-latin-400italic.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-400italic.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-500normal - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: normal;
-  font-display: swap;
-  font-weight: 500;
-  src:
-    local('Alegreya Sans Medium '),
-    local('Alegreya Sans-Medium'),
-    url(fonts/alegreya-sans-latin-500.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-500.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-500italic - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: italic;
-  font-display: swap;
-  font-weight: 500;
-  src:
-    local('Alegreya Sans Medium italic'),
-    local('Alegreya Sans-Mediumitalic'),
-    url(fonts/alegreya-sans-latin-500italic.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-500italic.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-700normal - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: normal;
-  font-display: swap;
-  font-weight: 700;
-  src:
-    local('Alegreya Sans Bold '),
-    local('Alegreya Sans-Bold'),
-    url(fonts/alegreya-sans-latin-700.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-700.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-700italic - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: italic;
-  font-display: swap;
-  font-weight: 700;
-  src:
-    local('Alegreya Sans Bold italic'),
-    local('Alegreya Sans-Bolditalic'),
-    url(fonts/alegreya-sans-latin-700italic.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-700italic.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-800normal - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: normal;
-  font-display: swap;
-  font-weight: 800;
-  src:
-    local('Alegreya Sans ExtraBold '),
-    local('Alegreya Sans-ExtraBold'),
-    url(fonts/alegreya-sans-latin-800.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-800.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-800italic - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: italic;
-  font-display: swap;
-  font-weight: 800;
-  src:
-    local('Alegreya Sans ExtraBold italic'),
-    local('Alegreya Sans-ExtraBolditalic'),
-    url(fonts/alegreya-sans-latin-800italic.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-800italic.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-900normal - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: normal;
-  font-display: swap;
-  font-weight: 900;
-  src:
-    local('Alegreya Sans Black '),
-    local('Alegreya Sans-Black'),
-    url(fonts/alegreya-sans-latin-900.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-900.woff) format('woff'); /* Modern Browsers */
-}
-
-/* alegreya-sans-900italic - latin */
-@font-face {
-  font-family: 'Alegreya Sans';
-  font-style: italic;
-  font-display: swap;
-  font-weight: 900;
-  src:
-    local('Alegreya Sans Black italic'),
-    local('Alegreya Sans-Blackitalic'),
-    url(fonts/alegreya-sans-latin-900italic.woff2) format('woff2'),
-    url(fonts/alegreya-sans-latin-900italic.woff) format('woff'); /* Modern Browsers */
-}
-
 table.swh-logos-table td,
 table.swh-logos-table th {
   text-align: center;