Add AI usage disclosure section to PR template

[aslamdoctor]

Summary

• Adds a new .github/PULL_REQUEST_TEMPLATE.md with structured sections (What, Why, How, Testing Instructions, Screenshots)
• Includes an AI Usage Disclosure section with checkboxes for contributors to indicate whether AI tools were used

Closes #1221

Test plan

• Verify the PR template renders correctly when creating a new PR
• Confirm the AI disclosure checkboxes are functional
• Ensure the template aligns with the WordPress AI Guidelines

[github-actions]

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Co-authored-by: aslamdoctor <aslamdoctor@git.wordpress.org>
Co-authored-by: raftaar1191 <raftaar1191@git.wordpress.org>

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