docs: add ai contributing policy

[chaokunyang]

Why?

Apache Fory is aperformance-critical foundational serialization framework.
As AI-assisted contributions increase, we need a project policy that keeps review quality high, protects legal/provenance safety, and preserves long-term maintainability.

This policy is intended to:

• keep human accountability as the core rule
• reduce low-signal or unverified AI-generated changes
• require verifiable testing/performance evidence for sensitive paths
• align contribution practice with ASF legal and governance expectations

What does this PR do?

• Adds a new top-level policy document: AI_CONTRIBUTION_POLICY.md.
• Defines requirements for AI-assisted contributions, including:
  • contributor responsibility and line-by-line self review
  • privacy-safe disclosure expectations
  • verification requirements (tests/spec/perf evidence where applicable)
  • licensing/provenance requirements
  • quality gate and maintainer enforcement process
• Updates .github/pull_request_template.md to add an AI contribution checklist for PR authors.

Related issues

N/A

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

Benchmark

N/A (documentation and PR template change only)

[pandalee99]

LGTM