Add num_proc to from_csv/generator/json/parquet/text
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5239). All of your documentation changes will be reflected on that endpoint. I ended up moving `num_proc` to `AbstractDatasetReader.__init__` :)

Let me know if it sounds good to you now
Allow multiprocessing to from_* methods