Automatically build doc notebooks
# What does this PR do?

This PR automatically build the notebooks associated to the doc pages and updates them when building the documentation. For now it relies on [this unmerged PR](https://github.com/huggingface/doc-builder/pull/50/files) on the doc-builder side, hence the specific branch (so I can test).

Once the PR there is merged we can remove the specific branch from the job.

As seen with @LysandreJik, I am going to merge this PR to test everything goes well and will revert if not :-)