`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?

Start in dm_control/mjcf/parser.py at the linked code around line 199 and trace how include elements are handled relative to the root element. Compare that behavior with MuJoCo's support for nested XML includes; done means includes work at any depth without breaking root-level parsing.

Written by the indexing model from the issue text.