[ONNX] Update user documentation (#85819)
- Remove mentions of `SymbolicContext` in the doc
- Comment out the PythonOp example so that it is not shown to users
- Updated code blocks and wording
- Changed to recommend using `pip` for installing onnx.
Now adds a deprecation message to the docs (demo only):
Fixes #85608
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85819
Approved by: https://github.com/AllenTiTaiWang, https://github.com/BowenBao
