[doc] DP/PP/TP/etc parallelism
This PR graduates the original notes from https://github.com/huggingface/transformers/issues/9766 combined with notes from 
@anton-l https://github.com/huggingface/transformers/issues/10321#issuecomment-783543530 and adding some extra materials into a separate doc.

Fixes: https://github.com/huggingface/transformers/issues/9766

