Version selector for Nix manual in title bar

[lf-]

Observations

Currently the Nix docs as seen here don't have the version number switchable and don't raise that you're not looking at the latest: https://nix.dev/reference/nix-manual

Problem

It's hard to navigate between Nix versions docs without knowing what the latest Nix version is.

Approaches

Consider docs.rs or docs.python.org as examples of the solution: there's a drop down in a top bar to select the version and on docs.rs there's a little warning if it's an old version.

This versioning thing could probably be reused for NixOS docs and such.

Willing to help?

Maybe

Priorities

Add 👍 to issues you find important.

[henrik-ch]

Further observation:
When navigating from the https://nixos.org/learn page to the manual, the link is https://nixos.org/manual/nix/stable/ .

On the other hand, when navigating from https://nix.dev/reference/nix-manual to 2.19, the link is https://nix.dev/manual/nix/2.19/
I would prefer if we could avoid having the same information available under different top urls - at least to me it's confusing.
On that note, I can't determine if those two links are fully equivalent.

[fricklerhandwerk]

@henrik-ch #830 is supposed to address that