Add minimal viable docs for addons

[plaindocs]

Address #11425

This is how I'm planning to handle the list of Addons. Add minimal changes to existing docs (I'm expecting some addons to need more than this) and link to them centrally from /addons.

How do we feel about this?

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:books: Documentation previews :books:

• User's documentation (docs): https://docs--11444.org.readthedocs.build/en/11444/

• Developer's documentation (dev): https://dev--11444.org.readthedocs.build/en/11444/