[Docs] Use lib.trivial.version to segment the docs

[MattSturgeon]

I have two ideas here:

1. The (new) homepage for the docs should have a link to the "other" version(s) of the docs
2. The versioned URL should (probably) use lib.trivial.release instead of "stable".

Available in nixpkgs lib.trivial, we have release ("24.11" or "24.05"), versionSuffix ("pre-git" or ".20241014.c0b1da3") and version (release + versionSuffix).

We could probably setup a redirect for stable -> latest release, although given we just have a static site it'd probably have to just be an HTML redirect.