[Quant][docs] Replace qconfig_dict with QConfigMapping in docs
Summary: https://github.com/pytorch/pytorch/pull/78452 replaced
qconfig_dict with QConfigMapping as the default API for prepare_fx,
prepare_qat_fx, and convert_fx. We should update the docs to reflect
this change as well.
Test Plan:
```
cd docs
make html
cd build/html
python -m server.http
```
Reviewers: jerryzh168, vkuzo
Subscribers: jerryzh168, vkuzo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78533
Approved by: https://github.com/vkuzo
