Version selector for Nix manual in title bar

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.

Start at the Nix manual page at https://nix.dev/reference/nix-manual and inspect how the title bar and documentation versions are presented. Compare the version-selection and outdated-version warnings on docs.rs or docs.python.org. Done means readers can select Nix manual versions and can tell when they are viewing an older version; consider whether the approach can be reused for NixOS documentation.