docs: ✏️ add links to the Datasets API
_The documentation is not available anymore as the PR was closed or merged._ OK, thanks @lhoestq. I'll close this PR, and come back to it with @stevhliu once we work on https://github.com/huggingface/datasets-server/issues/568
I added some links to the Datasets API in the docs. See https://github.com/huggingface/datasets-server/pull/566 for a companion PR in the datasets-server. The idea is to improve the discovery of the API through the docs.

I'm a bit shy about pasting a lot of links to the API in the docs, so it's minimal for now. I'm interested in ideas to integrate the API better in these docs without being too much. cc @lhoestq @julien-c @albertvillanova @stevhliu.