Update docs once dataset scripts transferred to the Hub

[albertvillanova]

Describe the bug

As discussed in:

• misc doc tweaks; will tag people on each change hub-docs#423 (review)

we should update our docs once dataset scripts have been transferred to the Hub (and removed from GitHub):

• [GH->HF] Part 2: Remove all dataset scripts from github #4974

Concretely:

• Datasets on GitHub (legacy): https://huggingface.co/docs/datasets/main/en/share#datasets-on-github-legacy
• ADD_NEW_DATASET: https://github.com/huggingface/datasets/blob/main/ADD_NEW_DATASET.md
• ...

This PR complements the work of:

• Fix CONTRIBUTING once dataset scripts transferred to Hub #5067

This PR is a follow-up of PRs:

• Start removing canonical datasets logic #3777

CC: @julien-c