Big model table
# What does this PR do?

This PR adds a big table in the first page of the doc, indicating whether each of our models has support for a slow/fast tokenizer, PyTorch, TensorFlow and Flax. Result can be found [here](https://125258-155220641-gh.circle-artifacts.com/0/docs/_build/html/index.html) (scroll a bit down).
It is updated automatically via `make fix-copies` and checked for updates in `make quality`, being built form the content of the auto models module.

There were a few issues with the imports on the flax side that I fixed in passing, and I renamed a constant to add the `FLAX` prefix. @mfuntowicz this doesn't really change anything but pinging you just so you're aware.