Add a banner allowing the user to redirect to a localized version

The language switcher on the bottom left is hardly discoverable for users not familiar with ReadTheDocs, as it's hidden by default.

I think it would benefit more users if we do some detection and provide the option to redirect to the user's preferred language on the docs page. For example, visiting might prompt you with a similar dialog if your browser's preferred language is not English.

I'm not sure if ReadTheDocs/Sphinx actually allows implementing this. But if it does, the logic might be something like this:

1. Check the `Accept-Language` header to see if we provide a version that matches the user's preferred language.
2. If such a version exists, show a banner (like the "Up to date" banner) that contains:
    * A message possibly in that language.
    * A link that redirects to the same page in that language.
    * A close button that prevents this banner from showing again, at least in this session.

See also https://github.com/godotengine/godot-proposals/issues/10538