`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?

从 dm_control/mjcf/parser.py 中 199 行附近的链接代码开始，跟踪相对于根元素如何处理 include 元素。将该行为与 MuJoCo 对嵌套 XML include 的支持进行比较；完成标准是 include 在任意深度都能工作，且不破坏根级解析。

由索引模型根据 Issue 内容生成。