Quantization docs: consilidate all API references on a single page (#66198)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66198
Consolidates all API reference material for quantization on a single
page, to reduce duplication of information.
Future PRs will improve the API reference page itself.
Test Plan:
```
cd docs
make html
python -m http.server
// renders well
```
Reviewed By: jerryzh168
Differential Revision: D31447616
Pulled By: vkuzo
fbshipit-source-id: 2f9c4dac2b2fb377568332aef79531d1f784444a
