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.