Styling of versions menu and warning banner

[ru-fu]

See https://about.readthedocs.com/blog/2024/07/addons-by-default/

The way the versions banner and the version warnings are displayed will change soon, which means that our custom styling (here ff) won't work anymore and will need to be redone.

We'll need those changes both here and synced to the canonical-sphinx package.

[ru-fu]

The current fly-out is misleading regarding the version:

It could mean the Read the Docs version instead of the documentation for a specific product version.