Add `flan-t5` documentation page
# What does this PR do?

This PR adds `FLAN-T5` on the documentation page - following the same approach for `t5-v1.1`: https://huggingface.co/docs/transformers/model_doc/t5v1.1 

cc @sgugger @ydshieh @ArthurZucker 

_The documentation is not available anymore as the PR was closed or merged._<|||||>Thank you for the feedback @sgugger ! I should have addressed the comments now 