Add banners and announcement bars

[choldgraf]

In Jupyter Book 1, users can create banners via their config file. These are displayed at the top of the site and are used to provide highly-noticeable messages to users. For example, below is a screenshot of the banner we use to tell users about the upcoming Jupyter Book 2:

Suggested functionality

The old implementation used raw HTML in the config to populate the banner, I think it'd be better if this used MyST markdown that was parsed at build time.

If straightforward, users should have the ability to dismiss the banner and have that propagate throughout pages (in both server- and static-forms of browsing)

Thought on priority

I think that this should be supported as part of a Jupyter Book 2 beta, if anything because we'll want that banner to give people an informative "heads up" once we make the switch for the latest branch on ReadTheDocs.