Add quantization documentation to vulkan docs

[metascroy]

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

If Vulkan does not currently support quantization, feel free to close this task

cc @SS-JIA @manuelcandales @cbilgin

[metascroy]

@SS-JIA can you have a look here for ExecuTorch 0.7?

[SS-JIA]

Acknowledged; will do.