“See also” section in manpages

[Profpatsch]

Many man pages have a SEE ALSO section cross-referencing other pages that could be interesting to the reader.

We should add them for nix-* and nixos-* as well.

[Profpatsch]

Supersedes NixOS/nixpkgs#19216, companion issue to NixOS/nixpkgs#42524

[stale]

I marked this as stale due to inactivity. → More info

[stale]

I closed this issue due to inactivity. → More info