Found using https://github.com/serokell/xrefcheck, which unfortunately
can't trivially be enforced in CI because we also have the manual markdown
files that need post-processing to be valid
Everything gets moved into the `ci/` top-level directory.
We keep behind `maintainers/scripts/check-by-name.sh` and `pkgs/test/check-by-name/pinned-version.txt` as they are going to cause CI errors and confusion until we get all the way through the various channels.
They'll be removed in about a week or so.
This is needed such that in the next commit, we can re-use the same
version from a shell.nix, allowing people to have a guaranteed matching
nixfmt version.
