Update onnx landing page for 1.3 (#27581)
Summary:
* Update supported operator list.
* Update FAQ on implicit scalar casting. Traced models are now more robust.
cc spandantiwari lara-hdr neginraoof Please feel free to add any missing points. Thank you!
cc houseroad for review.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27581
Reviewed By: hl475
Differential Revision: D17882147
Pulled By: houseroad
fbshipit-source-id: c1d745ca647fce2daf897bbb6d1ff8c283f18839
