chore(internal docs): add AI contribution policy (#25511)

* docs: add AI contribution policy

Define and publish an AI contribution policy and integrate it into the contributor workflow.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: reframe AI review thread guidance

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore: add documentation team as codeowner for AI_POLICY.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>