Add a `treefmt.optionsDoc` passthru, which is included on the treefmt
section of the nixpkgs manual.
This generates reference docs for options declared in
`treefmt.evalConfig`.
Add a `treefmt.functionsDoc` passthru, which is included on the treefmt
section of the nixpkgs manual.
This generates reference docs for functions defined in
`pkgs/by-name/tr/treefmt/lib.nix`, using `nixdoc`.