Update quantization overview doc page

[GregoryComer]

📚 The doc issue

The current quantization overview page is a bit sparse: https://pytorch.org/executorch/main/quantization-overview.html. I'd like to update it as follows:

• Move under Usage/ since it's the only page under Quantization/ currently.
• Split out information intended for backend authors (info about writing a quantizer, for example). Focus on user-facing APIs.
• Document backend-invariant quantization flows (embeddings, ao kernels, etc.). Include info (and example) on composable quantizer.
• Document PT2E and quantize_ flows.
• Cover the general, high level approach to quantizing different types of models.
  • CV models
  • Transformers / language models
• Talk briefly about options for evaluating quantized model accuracy (running in eager mode vs pybindings vs on-device, for example)

Suggest a potential alternative/fix

No response

cc @mergennachin @byjlw @kimishpatel @jerryzh168