Create a documentation page for FX graph mode quantization APIs (#66122)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66122
Description:
Adds a documentation page for FX graph mode quantization APIs which
reads from the docstrings in `quantize_fx`, and links it from the main
quantization documentation page.
Also, updates the docstrings in `quantize_fx` to render well with reStructuredText.
Test Plan:
```
cd docs
make html
python -m http.server
// open webpage, inspect it, looks good
```
Reviewed By: dagitses
Differential Revision: D31447612
Pulled By: vkuzo
fbshipit-source-id: 07d0a6137f1537af82dce0a729f9617efaa714a0
