Add installation instructions for NixOS, without Flakes or Home Manager

[Jaage]

It would be nice to add installation instructions to the documentation that don't rely on utilities like Flakes and Home Manager that are not native to NixOS.

[danth]

Unfortunately, the majority of Stylix's functionality is not possible without Home Manager due to applications only supporting configuration files within the home directory.

It's possible to use without flakes, though.