Add NixOS Matrix Distribution

### Project Affected

NixOS

### Current Information

NixOS is not listed on the distributions page

### Suggested Correction

Administrators can use NixOS Modules to setuo Matrix. Since modules can be composed, additional services can also be set up resulting in a distribution like experience

### Source

https://wiki.nixos.org/wiki/Matrix

### Additional Context

@HarHarLinks asked me to do this :)

Start by locating the distributions page in the website repository and read the NixOS Matrix source at https://wiki.nixos.org/wiki/Matrix. Add NixOS with an accurate description of using NixOS Modules to set up Matrix. Done means the distributions page lists NixOS and the documentation links to the cited source.

Written by the indexing model from the issue text.