Add page featuring affiliated packages and links to package documentation

[jonathansick]

From @mwcraig:

Quick thought as Brett Morris and I are working on the affiliated package section of Paper 3: can the new learn site index the short descriptions of each affiliated package to make them discoverable by search?
The affiliated package page https://www.astropy.org/affiliated/index.html is nice but not where folks might first go for help.

This is a great idea, and could be accomplished by adding another primary page to Learn Astropy featuring links to all packages in the Learn Astropy ecosystem.

A second level of integration would be to add records to Algolia that cover the package's metadata (titles, URLs, descriptions, keywords) so that this info could be more integrated into the Algolia UI for filtering, rather than being a static listing.

The next step would be to extend github.com/astropy/learn-astropy-librarian to index the full-text content of Sphinx documentation sites.