Export to ONNX doc refocused on using optimum, added tflite
Currently, despite the notes saying that the recommended way is to use Optimum, the main focus of the doc is on using `transformers.onnx`, which is no longer maintained (according to optimum team). 
This PR restructures the doc to put the optimum examples forward as the primary way for exporting models to ONNX. 
The example of using `transformers.onnx` is kept for potential compatibility reasons, however, I have removed the examples of adding new architectures to transformers.onnx, as I believe this should be done in Optimum instead (links provided).
As suggested by the team, I have also added an example for TFLite, and for that reason, renamed the page to "Export to ONNX, TFLite" 

UPD: since we now link to Optimum docs for the list of supported architectures, I have also removed the ONNX list check & update from the check_table script.
_The documentation is not available anymore as the PR was closed or merged._