NixOS installation tutorial?

[Definitely-Not-A-Dolphin]

There are flakes for NixOS in the project root, but not any docs on how to install it on NixOS. Maybe add that?

[FrankelJb]

There's a missing file for the nixos installer.

The install.sh script sets DISTRO_INSTALLER="$REPO_DIR/dots-extra/install-${DISTRO_TYPE}.sh" but then that file doesn't exist in this repo or any branch.