docs/community-builder: add example NixOS configuration for remote builds

[JohnRTitor]

Added example NixOS configuration for remote builds using nix-build and nix build commands.

Inspired from https://github.com/NixOS/aarch64-build-box?tab=readme-ov-file#configuring-your-computer-for-remote-builds

Tested in my configuration, now finally nixpkgs-review works!

[JohnRTitor]

Added a warning after discussing in the matrix channel.
I can't seem to fix the treefmt error in the CI though?

[zowoq]

Added example NixOS configuration for remote builds using nix-build and nix build commands.

Inspired from https://github.com/NixOS/aarch64-build-box?tab=readme-ov-file#configuring-your-computer-for-remote-builds

Added a warning after discussing in the matrix channel.

I intentionally didn't add this, didn't want to have an "example" distributedBuilds config, even with a warning.

now finally nixpkgs-review works!

Maybe we could show an example of passing builders to nixpkgs-review instead.

[JohnRTitor]

Maybe we could show an example of passing builders to nixpkgs-review instead.

I am not sure how nixpkgs-review works with remote builders actually.

But I thought it would be safe if we configure it where NixOS system platform != Builder platform