nixos/doc: add documentation for `formats.pythonVars`

    :   Outputs the xml with header.

`pkgs.formats.pythonVars` { }

:   A function taking an empty attribute set (for future extensibility)

    and returning a set with python variable specific attributes `type`, `lib`, and

    `generate` as specified [below](#pkgs-formats-result).

    The `lib` attribute contains functions to be used in settings, for

    generating special Python values:

    `mkRaw pythonCode`

    :   Outputs the given string as raw Python code

    `_imports`

    `_imports` is a special value you can set to specify additional modules to be

    imported on top of the file.

    `Example usage:`

    ```nix

      let

        format = pkgs.formats.pythonVars { };

      in {

        _imports = [ "re" ];

        conditional = format.lib.mkRaw "1 if True else 2";

        function_result = format.lib.mkRaw "re.findall(r'\\bf[a-z]*', 'which foot or hand fell fastest')";

      }

    ```

`pkgs.formats.cdn` { }

:   A function taking an empty attribute set (for future extensibility)