Fix and check docs

[roberth]

This makes sure that the docs can be rendered.

The error was as follows, excluding the stack trace:

error: Most nixpkgs attributes are not supported when generating documentation.
       Please check with `--show-trace` to see which option leads to this `pkgs.AAAAAASomeThingsFailToEvaluate` reference. Often it can be cut short with a `defaultText` argument to `lib.mkOption`, or by escaping an option `example` using `lib.literalExpression`.

[bors[bot]]

Build succeeded:

• check formatting [x86_64-linux]
• check testConfigGeneration [x86_64-linux]
• check testEmptyConfig [x86_64-linux]
• check testWrapper [x86_64-linux]
• Evaluate flake.nix