Document end-to-end quantization journey in ExecuTorch

### 🚀 The feature, motivation and pitch

1. Document how PTQ/QAT/quality validation looks in ET using the PT2E flow (the common flow implemented by all backends).

2. Document advanced research flows (e.g., GPTQ, PARQ) using quantize_.

### Alternatives

_No response_

### Additional context

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### RFC (Optional)

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