Add Datasets Card spec (#595)
* add datasetcard spec
* update dataset card docs
* Apply suggestions from code review
Co-authored-by: Julien Chaumond <julien@huggingface.co>
* add link to gating docs
* Apply suggestions from code review
Co-authored-by: Julien Chaumond <julien@huggingface.co>
* julien's comment
Co-authored-by: Julien Chaumond <julien@huggingface.co>
