docs: add Nix/NixOS installation details to README

[TheFacc]

Enhance README with Nix/NixOS installation details

Description
Added installation instructions for Nix and NixOS, including usage with and without flakes.

Motivation and Context
Only editing README.md as agreed in a previous PR, no repo flake.
Used toggles to avoid taking visual space for non-Nix users.
The instructions are not tied to any version, the specified versions/revs are just examples, everything will keep working in future repo updates.

Checklist:

• I have written unit tests for my changes.
• I have updated the documentation accordingly.
• All new and existing tests passed.

[Yakitrak]

Thanks for the follow-up! The details toggles are a nice touch, but this is still quite a bit longer than what I had in mind, the other install sections are just a couple of commands each.

Could you please trim this down to something closer in size to the AUR or Homebrew sections? If thats not possible, as I said before, its a sign we should add to nixpkg, and then update instructions. This is what we did for AUR, @jneidel kindly added the configuration for it and then we updated instructions.

[jneidel]

Suggested change

[jneidel]

Suggested change

[jneidel]

The currently form is only really useful for people already very familiar with Nix.
A nix package will be a better approach.

Until then this documentation is available if one searches the repo for "nix", so it is accessible for the small nix crowd that is looking for it.