Where in git is the NixOS manual? (want to submit a pull request)

I’d like to make a minor improvement to some language in the NixOS manual and submit a pull request. However, I’m git grepping for it in what I think is the git repo for it, but not finding it. Can someone point me to either the stable or unstable NixOS manual page in git? Thanks!

You can find some information in NixOS - NixOS 21.11 manual.