Add URL field to Wikimedia dataset instances: wikipedia,...
As reported by @geohci, once we will host pre-processed data in the Hub, we should add the full URL to data instances (new field "url") in order to conform to proper attribution from license requirement. See, e.g.: https://fair-trec.github.io/docs/Fair_Ranking_2021_Participant_Instructions.pdf#subsection.3.2

This should be done for all pre-processed datasets under "wikimedia" org in the Hub: https://huggingface.co/wikimedia
