Add quantization documentation to the Qualcomm docs

[metascroy]

The Qualcomm backend docs do not discuss how to do quantization: https://github.com/pytorch/executorch/blob/main/docs/source/backends-qualcomm.md

From the backend-template, the quantization section should document how to quantize a model and go over the available quantization configs: https://github.com/pytorch/executorch/blob/main/docs/source/backend-template.md#quantization

See examples for XNNPACK and CoreML.

Once the section is filled out, add a link to the quantization overview page.

cc @cccclai

[metascroy]

@cccclai can you have a look for ExecuTorch 0.7 release?

[cccclai]

The doc is updated here #12387