Start removing canonical datasets logic
I'm not sure if the documentation explains why the dataset identifiers might have a namespace or not (the user/org): 'glue' vs 'severo/glue'. Do you think we should explain it, and relate it to the GitHub/Hub distinction? > I'm not sure if the documentation explains why the dataset identifiers might have a namespace or not (the user/org): 'glue' vs 'severo/glue'. Do you think we should explain it, and relate it to the GitHub/Hub distinction?

I added an explanation, let me know if it sounds good to you:

```
Datasets used to be hosted on our GitHub repository, but all datasets have now been migrated to the Hugging Face Hub.
The legacy GitHub datasets were added originally on our GitHub repository and therefore don't have a namespace: "squad", "glue", etc. unlike the other datasets that are named "username/dataset_name" or "org/dataset_name".
```
 Thanks for the feedbacks ! Merging this now - if you have some comments I can take care of them in a subsequent PR

I'll also take care of resolving the conflicts with https://github.com/huggingface/datasets/pull/3690
I updated the source code and the documentation to start removing the "canonical datasets" logic.

Indeed this makes the documentation confusing and we don't want this distinction anymore in the future. Ideally users should share their datasets on the Hub directly.

### Changes

- the documentation about dataset loading mentions the datasets on the Hub (no difference between canonical and community, since they all have their own repository now)
- the documentation about adding a dataset doesn't explain the technical differences between canonical and community anymore, and only presents how to add a community dataset. There is still a small section at the bottom that mentions the datasets that are still on GitHub and redirects to the `ADD_NEW_DATASET.md` guide on GitHub about how to contribute a dataset to the `datasets` library
- the code source doesn't mention "canonical" anymore anywhere. There is still a `GitHubDatasetModuleFactory` class that is left, but I updated the docstring to say that it will be eventually removed in favor of the `HubDatasetModuleFactory` classes that already exist

Would love to have your feedbacks on this !

cc @julien-c @thomwolf @SBrandeis 