Remove instructions 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 