Paperswithcode dataset mapping
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
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
