Quantization documentation: move backend section down (#66210)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66210
Description:
Moves the backend section of the quantization page further down,
to ensure that the API description and reference sections are closer
to the top.
Test Plan:
```
cd docs
make html
python -m server.http
// renders well
```
Reviewed By: jerryzh168
Differential Revision: D31447611
Pulled By: vkuzo
fbshipit-source-id: 537b146559bce484588b3c78e6b0cdb4c274e8dd
