Provide a way to easily run doc tests

[srid]

https://github.com/numtide/treefmt-nix/pull/86

One day I'll add CI for the docs; need to work on the OSS use case a bit.

Something that runs doc tests just for the current module would be nice, rather than all modules.

Maybe as a flake-module ...

{
  imports = [
    flake-parts.flakeModules.doctest
  ];
  # adds a flake check
  flake.flake-parts.doctest.modulesToTest = [ self.flakeModule ]; # default anyway
}

[srid]

Maybe as a flake-module ...

Actually, that won't work since the flake containing the flake module doesn't use flake-parts itself.