Support ndjson data files
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7154). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update. Thanks for your review, @severo.

Yes, I was aware of this. From internal conversation:
> Please note that although NDJSON was planned to be submitted as an RFC standard spec, it is no longer maintained:
> - See note from the author: https://github.com/ndjson/ndjson-spec/issues/35#issuecomment-1285673417
> - See that their official website domain has expired: https://ndjson.org/ 

The purpose of this PR is just supporting datasets with ndjson data files (e.g. Wikimedia Enterprise data files), but it should not imply any recommendation or endorsement of this format from our part.
Support `ndjson` (Newline Delimited JSON) data files.

Fix #7153.