[ViTMAE] Various fixes
# What does this PR do?

This PR:
- adds a link to a demo notebook for Facebook's MAE.
- fixes the links to the docs of various Vision Transformer-based models in ViT's doc page.
- fixes the code examples for MAE.
- adds MAE to the `AutoFeatureExtractor` API.
Great job merging this PR! the documentation will now be removed from the staging environment.