Announcement: allow users to publish notes on their docs

[humitos]

I remember talking to someone from the core team about exposing the ability to make "announcements" to project maintainers on their documentation. I'm thinking about something similar to what Webpack documentation is doing.

The "announcement addons" could be used to publish news or similar in all the pages of the project's documentation. I haven't thought too much about this and the idea could be expanded after some brainstorming, but I think it has potential. Definitely, pretty useful if you want to add these announcements to old versions without rebuilding them. Users should be able to configuration the "versions affected" and also the "message" to show.

Live example: https://webpack.js.org/concepts/

[humitos]

There is some ideas we can get from this Material for MkDocs conversation squidfunk/mkdocs-material#5803

[humitos]

This could now be implemented by the user itself using the CustomScript addon: readthedocs/readthedocs.org#11758