Promote these materials throughout Astropy ecosystem

The notebooks in this repo are extremely useful and yet very difficult to discover. We need to add links here from
- astropy.org, get help page
- learn.astropy.org - maybe make an entry per lesson?
- documentation for each subpackage.

The issue names astropy.org's get-help page, learn.astropy.org, and each subpackage's documentation as targets, but no repository files or tests. Start by locating how those pages and docs are maintained and inventory the workshop notebooks and lessons; done means the materials are linked from each agreed location without broken links.

Written by the indexing model from the issue text.