Pass datasets-cli additional args as kwargs to DatasetBuilder in `run_beam.py`
Hi,

Following this <https://discuss.huggingface.co/t/how-to-preprocess-a-wikipedia-dataset-using-dataflowrunner/41991/3>, here is a simple PR to pass any additional args to datasets-cli as kwargs in the DatasetBuilder in `run_beam.py`.

I also took the liberty to add missing setup steps to the `beam.mdx` docs in order to help everyone.

@lhoestq 