Document group id parameter.

Documentation/ProjectFile/prj/parameters/parameter/Group/c_Group.md

+Group parameter (also known as a material dependent parameter) defines a
+specific value or values for each of the group ids.
+
+The group_id_property specifies the field in the mesh (MeshLib::Properties) from
+which the group ids are used.

Documentation/ProjectFile/prj/parameters/parameter/Group/t_group_id_property.md

+\ogs_missing_documentation