Improve documentation of some models (#14695)
* Migrate docs to mdx
* Update TAPAS docs
* Remove lines
* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* Apply some more suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* Add pt/tf switch to code examples
* More improvements
* Improve docstrings
* More improvements
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>