[docs] make dummy data creation optional
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_3894). All of your documentation changes will be reflected on that endpoint. The dev doc build rendering doesn't seem to be updated with my last commit for some reason Merging it anyway since I'd like to share this page with users 🙃 
Related to #3507 : dummy data for datasets created on the Hugging Face Hub are optional.

We can discuss later to make them optional for datasets in this repository as well