Non-latest version warning: implement a theme-agnostic banner

[humitos]

Our current non-latest version warning is pretty nice integrated with the Read the Docs Sphinx theme and that's fine.

However, we need a generic integration that works good for all the styles. This could be achieved by injecting a fixed banner instead, like the one that's used by Flask:

I think the default banner should be the generic one and theme authors should be able to define where this chunk of HTML will be injected by defining a <div id="non-latest-version-warning"></div> in case they want to integrate it in a nicer way. Similar to what we do with the flyout (https://docs.readthedocs.io/en/stable/flyout-menu.html)

(grabbed from readthedocs/readthedocs.org#9755 (comment))

[humitos]

This will probably follow the same style than #18. I think we will reuse the HTML structure and CSS style for this banner as well.

[humitos]

We already decide how we are going to do this. There is a lot of progress in #37. We will re-use that idea to do this warning as well.