Update documentation: Add adrf third party package

[maerteijn]

Description

Updated documentation: Added adrf as a third party package and updated the API documentation to refer to the async counterparts of:

• @api_view
• APIView
• Serializer
• ModelSerializer
• BaseSerializer
• ListSerializer
• ViewSet

See also #8496

[auvipy]

thanks, i need to think about the wording and placing them properly

[tomchristie]

We should link to adrf, but I'm not convinced we should be doing that in a way that's different to our existing third party packages. (I don't think we have other cases where we're interspersing links to a third party package throughout the docs?)

Perhaps we could just add a section in https://www.django-rest-framework.org/community/third-party-packages/ (?)

# Existing Third Party Packages

...

## Async Support

* adrf - Provides async Views, ViewSets, and Serializers.

## Authentication

...

Optionally it could also be included in one or more of the existing pages "Third Party Packages" sections.

[maerteijn]

Updated the PR as suggested by @tomchristie with just a single reference to adrf (on the third party packages page).