Cortex-M: Add backend documentation to docs site

[pytorchbot]

Summary

Adds the Cortex-M backend overview page to the ExecuTorch documentation website, making it discoverable alongside other embedded backends. The page covers target support, CMSIS-NN operator table, quantization, and a tutorial walking through export, quantization, edge lowering, and cross-compilation.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18420

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 5 New Failures, 2 Cancelled Jobs, 1 Unrelated Failure

As of commit 4f41cc9 with merge base 8c0a60b ():

NEW FAILURES - The following jobs have failed:

• pull / test-moshi-linux / linux-job (gh)
  RuntimeError: Command docker exec -t e332f354d5b0a871e87f73e0bec19b27ea3828a09857897534460930e0690646 /exec failed with exit code 1
• pull / test-voxtral-realtime-xnnpack-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 70404ede6da4f5801cf539fff6bb24abb9d12f6f26c7bb07cd2a114f6717ad83 /exec failed with exit code 1
• pull / unittest-arm-backend-with-no-deps (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 54b20cbc9ebc10ae53fb1ef97afe9f52e2f3a11702095fbdddba58571f346597 /exec failed with exit code 1
• Test CUDA Builds / check-all-cuda-builds (gh)
  Process completed with exit code 1.
• Test CUDA Builds / test-executorch-cuda-build-12.6 / linux-job (gh)
  RuntimeError: Command docker exec -t a946d9519a730391ca15f5ea80126802ca5800e74c1942fd0a03ea4478d648be /exec failed with exit code 1

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / unittest / macos / macos-job (gh)
  ##[error]The operation was canceled.
• pull / unittest-editable / macos / macos-job (gh)
  ##[error]The operation was canceled.

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• Test CUDA Builds / test-model-cuda-e2e (google, gemma-3-4b-it, non-quantized) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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