[WIP] Prototype updated XNNPACK doc structure

[GregoryComer]

Summary

Prototype an updated doc structure for the XNNPACK backend.

This PR updates the doc structure as follows:

• XNNPACK Backend
• Reference
  • Quantization
  • Partitioner APIs
  • Architecture and Internals
• Tutorials
  • Basic tutorial (name TBD)

[pytorch-bot]

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As of commit 0c47d79 with merge base 5dee222 ():

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• pull / test-samsung-models-linux / linux-job (gh)
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[GregoryComer]

This is a pretty early draft, but I'd be interested in any thoughts.
https://docs-preview.pytorch.org/pytorch/executorch/14873/backends/xnnpack/xnnpack-overview.html

CC @digantdesai @psiddh @mergennachin @cccclai @metascroy