Arm backend: Update backends/arm/README.md

[tom-arm]

• Rewrite to make it more concise

Change-Id: I57c5dbc8f87a4e8f527597ec721d9697f41e5d3b

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14206

• 📄 Preview Python docs built from this PR

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

❌ 1 New Failure, 6 Unrelated Failures

As of commit cbe94ee with merge base 2805a81 ():

NEW FAILURE - The following job has failed:

• trunk / test-models-macos-mps / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 139

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

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

• Build Windows Wheels / pytorch/executorch / build-wheel-py3_10-cpu (gh) (trunk failure)
  RuntimeError: Failed to install QNN SDK. Please check the logs above.
• Build Windows Wheels / pytorch/executorch / upload / upload-wheel-py3_10-cpu (gh) (trunk failure)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_x64
• pull / test-binary-size-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-moshi-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-setup-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[zingo]

@digantdesai is this ok to merge when CI is in better shape?