Add installation over NixOS

[Chemaclass]

Recently, we managed to install phel:0.16 on NixOS

• phel: 0.13.0 -> 0.16.0 NixOS/nixpkgs#314348 by @drupol

The task is about adding the information how to use phel within nixos, either in an installation page or better in a new section within the getting started page: https://phel-lang.org/documentation/getting-started/

[drupol]

I'll review this at the end of the day, I'm not supposed to work on nix related stuff during working hours.

[Chemaclass]

Absolutely, no rush! Let me know if you need any help for this, @drupol 😃