doc: generate documentation for lib.customisation

      { name = "sources"; description = "source filtering functions"; }

      { name = "cli"; description = "command-line serialization functions"; }

      { name = "gvariant"; description = "GVariant formatted string serialization functions"; }

      { name = "customisation"; description = "Functions to customise (derivation-related) functions, derivatons, or attribute sets"; }

    ];

  };