Add Metal backend documentation to Voxtral README

[manuelcandales]

This PR updates the Voxtral README to document Metal backend support on Apple Silicon.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15273

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❗ 1 Active SEVs

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❌ 4 New Failures, 3 Unrelated Failures

As of commit db8eecd with merge base 7d4db45 ():

NEW FAILURES - The following jobs have failed:

• Build Windows Wheels / pytorch/executorch / build-wheel-py3_10-cpu (gh)
• Build Windows Wheels / pytorch/executorch / upload / upload-wheel-py3_10-cpu (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_x64
• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  RuntimeError: Command docker exec -t 78c99cd1243911e1732f9ad5683483e7cb510807ae88ce4454cbc6f5f85839b5 /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 6f596ed658dff7d1c9c4c775c6b6834eba7ab3e8b9358f3d20013e12be16719a /exec failed with exit code 1

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

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

• pull / android / run-emulator (gh) (trunk failure)
  The process '/usr/bin/sh' failed with exit code 255
• pull / unittest / linux / linux-job (gh) (trunk failure)
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)

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