quantization: improve documentation on natively supported backends (#58925)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58925
Cleans up documentation on natively supported backends. In particular:
* adds a section title
* deduplicates information about fbgemm/qnnpack
* clarifies what `torch.backends.quantized.engine` does
* adds code samples with default settings for `fbgemm` and `qnnpack`
Test Plan: Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D28681840
Pulled By: vkuzo
fbshipit-source-id: 51a6ab66934f657553351f6c84a638fd5f7b4e12
