I don't use NixOS but I have a similar documenting approach to all my OS and $HOME config files. Every customized file / override is checked in to a git repo, and there's an `install.sh` that takes each such file / directory and copies / symlinks it from the repo to its target location. Basically the same concept as dotfiles, but also for system files. I've restored a couple of machines from OS reinstall to full readiness this way.