Add Intel macOS check; update docs

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Summary

Fixes #9772

Adds a platform check in install_requirements.py to exit early on Intel-based macOS systems since they're no longer supported by PyTorch/ExecuTorch. Also updates documentation.

References:

• Cannot build Apple frameworks on Intel Mac (comment)
• PyTorch macOS x86 builds deprecation (January 2024)

Test plan

• macOS (ARM64): Verified install proceeds normally
• macOS (Intel): Verified install_requirements script exits with ERROR: ExecuTorch does not support Intel-based macOS platforms. message

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/9865

• 📄 Preview Python docs built from this PR

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

✅ No Failures

As of commit 2b067cf with merge base 0844c38 ():
💚 Looks good so far! There are no failures yet. 💚

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@pytorchbot label "topic: not user facing"

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@pytorchbot unlabel "topic: not user facing"

[pytorch-bot]

❌ 🤖 pytorchbot command failed:

@pytorchbot: error: argument command: invalid choice: 'unlabel' (choose from 'merge', 'revert', 'rebase', 'label', 'drci', 'cherry-pick', 'close')

usage: @pytorchbot [-h] {merge,revert,rebase,label,drci,cherry-pick,close} ...

Try @pytorchbot --help for more info.

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@pytorchbot label "module: build/install"