[ViLT] Add link to notebooks
# What does this PR do?

Add link to my ViLT notebooks in the docs.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15791). All of your documentation changes will be reflected on that endpoint.<|||||>I think we should consolidate all the notebooks linked from the documentation inside the [notebooks repository](https://github.com/huggingface/notebooks) (apart from the community page of course). Would you mind mirroring your notebook there, with a clear line at the top "This notebook is a copy of the notebook developed by @NielsRogge [here](linked to notebook in your repo]"?<|||||>Can I keep those in my repo Transformers-Tutorials and link to that repo instead?

I'm kind of free with the format there, it's not really following the official example notebooks (I'm mostly using native PyTorch for training).