Improve documentation of some models
# What does this PR do?

This PR:
- migrates the docs of ImageGPT, TAPAS and TrOCR from rst to Markdown. 
- adds links to my demo notebooks for ViT, BEiT, LUKE, Perceiver.
- improves the docstrings of the Perceiver model.
Please ping me when you're ready to merge it @NielsRogge so that I may update the stable docs with this