Update Wikipedia dataset
_The documentation is not available anymore as the PR was closed or merged._ I'm starting to generate the pre-processed data for some of the languages (for backward compatibility).

Once this merged, we will create the pre-processed data on the Hub under the Wikimedia namespace. All steps have been properly done.

I'm merging all these commits into master.
This PR combines all updates to Wikipedia dataset.

Once approved, this will be used to generate the pre-processed Wikipedia datasets.

Finally, this PR will be able to be merged into master:
- NOT using squash
- BUT a regular MERGE (or REBASE+MERGE), so that all commits are preserved

TODO:
- [x] #3435
- [x] #3789
- [x] #3825
- [x] Run to get the pre-processed data for big languages (backward compatibility)
- [x] #3958

CC: @geohci