Update Wikipedia dataset
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
_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.