Enable announcement banner to be sticky

[alexisthual]

What's happening?

Currently, I use the announcement banner to warn users that they are looking at an outdated version of the generated docs. However, the banner is not sticky / fixed, so it disappears when users scroll down the page. This can be problematic when users access the docs through a hyperlink referencing a paragraph, as they probably won't see the banner.

I tried using custom CSS to simply make the banner sticky, but it seems it seems more complex (see discussion #545). In particular, the rest of the page will scroll under the banner, eventually hiding the logo, which I think is not the expected behaviour.

Reproducer

Simply use announcement option and scroll down.

Expectation

In my opinion, banners should be sticky by default in order to prevent this, but I guess having an option to make them sticky or not could be nice! :slightly_smiling_face:

Code of Conduct

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