Add a banner on outdated 4.x branch versions

[Calinou]

Your Godot version: 4.5.1

Issue description: The website currently doesn't display a banner when you're browsing an old version's documentation (e.g. 4.4 while 4.5 is already out). This leads to users regularly browsing outdated documentation, and not realizing this fact (example).

This is especially the case now that our cherry-picking policy to previous versions is stricter. We don't cherry-pick quality-of-life improvements as often as before to avoid breaking translations and reduce maintenance churn.

Branches until 4.3 (inclusive) had this banner since many pages were still written for Godot 3, but it was removed as nearly all pages are now up-to-date:

We can reintroduce a similar banner on each branch that isn't the latest stable version, so that all pages have this notice. This can be implemented either as a static banner on each branch, or using some JavaScript code to dynamically insert a banner according to the current version. (I'm not sure which approach should be preferred here.)

It could look like this:

The link would point to the /stable branch of the documentation. This banner would never appear on localized pages, as they use a singular 4.x branch.

URL to the documentation page: N/A, as this applies to all documentation pages