docs: add AI usage guide

[kmishra1204]

Closes #21.

Summary

• Adds docs/ai-usage-guide.md for AI-assisted taxonomy, landscape, issue, PR, and documentation contributions.
• Links the guide from CONTRIBUTING.md so contributors see it before drafting or reviewing content.
• Covers human ownership, source verification, privacy boundaries, prohibited uses, PR disclosure, and reviewer checks.

Scope note

This is contribution guidance only. It does not claim AAIF or Linux Foundation endorsement, does not create a binding foundation-wide AI policy, and does not change taxonomy or landscape data.

Validation

• git diff --check
• ASCII scan for docs/ai-usage-guide.md
• Confirmed existing non-ASCII characters in CONTRIBUTING.md were pre-existing and the added link line is ASCII

[kmishra1204]

Validated this current head after fetching upstream main.

Local checks:

• Branch is one commit ahead and zero behind upstream main
• git diff --check origin/main...fork/codex/ai-usage-guide-issue-21
• Changed-file scope is markdown-only: docs/ai-usage-guide.md, CONTRIBUTING.md
• Relative links in changed markdown resolve inside the branch
• No TODO/FIXME/example placeholder text found in changed files