[Docs] How are the 3D bounding boxes defined?

### Branch

main branch  https://mmdetection3d.readthedocs.io/en/latest/

### 📚 The doc issue

I'm curious about how the 3D bounding boxes are defined – are they oriented bounding boxes (OBB), or axis-aligned boxes vertical to the ground plane? If possible, could you share or reference the code for that?



### Suggest a potential alternative/fix

_No response_