Paperswithcode dataset mapping
This is a continuation of https://github.com/huggingface/huggingface_hub/pull/43, encoded directly inside dataset cards.

As discussed:
- `paperswithcode_id: null` when the dataset doesn't exist on paperswithcode's side.
- I've added this new key at the end of the yaml instead of ordering all keys alphabetically as pyyaml's default. No strong opinion on that one though

messed up my branch, repushing live mapping can be found at https://huggingface.co/api/pwc/datasets-mapping and will be kept up to date going forward