`mjcf` parser only supports `include` at root level

I think the `mjcf` parser [assumes](https://github.com/google-deepmind/dm_control/blob/main/dm_control/mjcf/parser.py#L199) that `include` elements can only be children of the root element, while, in contrast, MuJoCo allows `include` to live at any depth in the XML tree?