chore: add link to the video cls notebook.
We recently added a [notebook](https://github.com/huggingface/notebooks/blob/main/examples/video_classification.ipynb) that shows how to fine-tune the VideoMAE model on a custom dataset. This PR adds the notebook link to the model doc. 

Cc: @osanseviero 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@NielsRogge I don't have the rights to merge the PR. If you have, could you perform the duties? 