user: Bootstrap loaders high-level documentation

Add landing page of loaders high-level documentation displaying available ones in a table with the following column:

• logo and name of the loader linking to its specific documentation

• related links: source code, developer doc, development activity

• current status: production, staging or development

• link to related grant if any

Create one rst file per loader to further document.

Related to #3117

---

Migrated from D7637 (view on Phabricator)

docs/_static/custom.css

@@ -378,4 +378,18 @@ a {
     local('Alegreya Sans-Blackitalic'),
     url(fonts/alegreya-sans-latin-900italic.woff2) format('woff2'),
     url(fonts/alegreya-sans-latin-900italic.woff) format('woff'); /* Modern Browsers */
-}
\ No newline at end of file
+}
+
+table.swh-logos-table td,
+table.swh-logos-table th {
+  text-align: center;
+}
+
+table.swh-logos-table img {
+  margin-bottom: 5px;
+}
+
+table.swh-logos-table ul {
+  text-align: left;
+  margin-bottom: auto;
+}