[Docs] Fix Apple silicon include path in CPU installation docs

[sjhddh]

Summary

• Fix incorrect include path in the CPU installation documentation
• The "Build image from source" section for Apple silicon was referencing cpu.arm.inc.md instead of cpu.apple.inc.md

Test Plan

• Build the documentation locally with mkdocs build or mkdocs serve
• Verify the Apple silicon section renders correctly without include errors

Risk

• Very low risk: documentation-only change
• Rollback: simple revert if needed

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[mergify]

Documentation preview: https://vllm--32977.org.readthedocs.build/en/32977/

[gemini-code-assist]

Code Review

This pull request correctly fixes an include path in the CPU installation documentation for Apple silicon. By pointing to cpu.apple.inc.md instead of cpu.arm.inc.md, it ensures that the correct content is displayed for users on Apple silicon. This is a good documentation fix that improves accuracy. The change is correct and ready to be merged.