Update CoreML docs

[metascroy]

This updates CoreML docs to:

• Discuss the new partitioner options
• Discuss quantize_ support
• Discuss backward compatibility guarantees

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13120

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❗ 1 Active SEVs

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• ghstack-mergeability-check and Check labels failing with 'Resource not accessible by integration'

❌ 3 New Failures, 1 Unrelated Failure

As of commit a1fce54 with merge base ec35f56 ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 255
• pull / test-moshi-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 32c007eef732a243ada13390477690f2cdbb45d335c5af099101cdca51c5ce7c /exec failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  AttributeError: '_OpNamespace' 'quantized_decomposed' object has no attribute 'convert_element_type'

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