Remove instructions to generate dummy data from our docs
Note that for me personally, conceptually all the dummy data (even for "canonical" datasets) should be superseded by `datasets-server`, which performs some kind of CI/CD of datasets (including the canonical ones) I totally agree: next step should be rethinking if dummy data makes sense for canonical datasets (once we have datasets-server) and eventually remove it.

But for now, we could at least start by removing the indication to generate dummy data from our docs.
In our docs, we indicate to generate the dummy data: https://huggingface.co/docs/datasets/dataset_script#testing-data-and-checksum-metadata

However:
- dummy data makes sense only for datasets in our GitHub repo: so that we can test their loading with our CI
- for datasets on the Hub:
  - they do not pass any CI test requiring dummy data
  - there are no instructions on how they can test their dataset locally using the dummy data
  - the generation of the dummy data assumes our GitHub directory structure:
    - the dummy data will be generated under `./datasets/<dataset_name>/dummy` even if locally there is no `./datasets` directory (which is the usual case). See issue:
      - #4742 

CC: @stevhliu 