[Docs] Top-level documentation of interdependent features

📚 The doc issue

Due to the project specific documentation, there isn't information about interdependent parts. If I am using a CCN from MMCV, but everything else it in MMDet or MMEngine, I have a hard time telling if there were other ways of getting the model object type, especially because the class is just initializing PyTorch provided resources.
https://mmcv.readthedocs.io/en/2.x/_modules/mmcv/cnn/utils/fuse_conv_bn.html#fuse_conv_bn

Suggest a potential alternative/fix

It would be helpful to have an interface or relevant links for where those resources can be found in other packages. I think MMEngine should be the location for this connection as it has been made into the workhorse for the rest of the packages.

Start with the linked MMCV fuse_conv_bn documentation and review how related resources are exposed across MMCV, MMDet, MMEngine, and PyTorch. Map the relevant interfaces or links between these packages; the documentation is done when users can find alternative ways to obtain the model object type and understand which package owns each resource.