[ONNX] Minor doc update (#69501) (#69550)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69550
Fix the wiki URL.
Also minor reorganization in onnx.rst.
Test Plan: Imported from OSS
Reviewed By: msaroufim
Differential Revision: D32994269
Pulled By: malfet
fbshipit-source-id: 112acfe8b7c778d7e3c2cef684023fdaf2c6ec9c
(cherry picked from commit f0787fabde906bdd9e14fb95cee5ebdfc770f75b)
