* Updated ONNX documentation
First draft on the documentation explaining how to export SB3 models in the ONNX format
* Updated changelog with ONNX documentation fix
* Address comments
* Update changelog.rst
* Update rtd env
* Fixes + add test example
Co-authored-by: Antonin RAFFIN <antonin.raffin@ensta.org>
Co-authored-by: Anssi Kanervisto <anssk@Anssis-MacBook-Air.local>
Co-authored-by: Anssi Kanervisto <kaneran21@hotmail.com>