Currently, our logic to show a notification is:

• on latest show a warning about potentially unreleased features
• on non-stable show a warning about maybe reading an outdated documentation

We need to handle the real branch/tag names these stable / latest versions refer to. Why is this? Because we want to show the latest notification on main too, for example. Also, we don't want to show the non-stable notification if the stable version points to v3.1 and the user is reading the v3.1.

The API should return a list of these alias, like:

{
  "latest": ["latest", "main"],
  "stable": ["stable", "v3.1],
}

and the JS code should check the current version against those.

Related #125