Removed format exclusion exceptions for modules that are already
formatted correctly (that is, when running nixfmt, no changes happen) or
have been moved (in the case of i3.nix).
The format script can be used to automatically format the Nix source
files and also verify that the files are formatted using the `-c`
command argument.
At the moment some files are exempt from the formatting to avoid
causing merge conflicts in active pull requests.
Finally, update the contribution guidelines to note that `nixfmt`
should be used.