Clean up ET CoreML documentation

[metascroy]

🚀 The feature, motivation and pitch

[P0] Quantization flow should be better documented, along with examples.
[P0] Supported compile specs in the partitioner should be documented, along with examples.
[P0] Document using the ET SDK with CoreML.
[P1] Debugging tools like executorch/examples/apple/coreml/scripts/extract_coreml_models.py should be documented, along with examples.
[P1] Where possible, CoreML ET delegate should reuse examples in ET codebase. For example, CoreML should work with EXECUTORCH_BUILD_EXECUTOR_RUNNER, rather than use its own runner in examples/coreml/apple. This will provide a more uniform user experience across delegates.

Alternatives

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Additional context

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RFC (Optional)

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[metascroy]

Most documentation was improved in #9463.

The EXECUTORCH_BUILD_EXECUTOR_RUNNER now supports CoreML with: #9094 (but this is not documented)

Remaining items:

• Documenting the SDK
• Documenting subbyte quantization when supported
• Documenting QAT when supported