Add local and TensorFlow ONNX export examples to docs
# What does this PR do?

This PR builds on #13831 to show examples of:

* Exporting a local checkpoint to ONNX
* Exporting a pure TensorFlow checkpoint to ONNX

_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for the tip @sgugger - I'd totally forgotten about this neat feature of the docs :) I've now implemented the change, so should be good for another pass 