DOC: add placeholder for announcement banner in docs (and use for 3.0rc announcement)

[jorisvandenbossche]

Would have to refine the actual message for right now, but general idea is that it would look like this:

The banner disappears once you scroll down, but is otherwise always visible (there is no "x" to click it away). You can see a live version of it on the theme docs: https://pydata-sphinx-theme.readthedocs.io/en/stable/index.html

Thoughts?

[jorisvandenbossche]

And because it is set up with a URL (not hardcoded text in the docs itself), once this is included on the hosted docs, you can change or disable (with empty file) the banner by changing the template file in the main repo, without having to rebuild the docs.