Improve vision models docs
# What does this PR do?

Improves the docs of several vision models:
* for `xxxForMaskedImageModeling` models, add a link to the `run_mim.py` script
* for `ViTMAEForPreTraining`, add a link to the `run_mae.py` script
* for ViT, add a tip about interpolation of pre-trained position embeddings (in order to fine-tune on higher resolution images)
* add figures for ViT and BEiT
_The documentation is not available anymore as the PR was closed or merged._