[make docs] parallel build
This PR enables multi-worker doc building.

After experimenting with different number of workers https://github.com/huggingface/transformers/issues/9496#issuecomment-758145868 4-5 workers seems to be the most optimal - let's go with 4 as surely we wouldn't find a cpu with less cores these days.

Fixes part of https://github.com/huggingface/transformers/issues/9496

@sgugger
