Add AI contribution guidelines

[stuartmorgan-g]

Adds a section about expectations around using AI tools to the contributor docs.

Also adds a PR checklist item calling them out (although like the first checklist item they are a subset of the overall docs that the next entry links to).

[gemini-code-assist]

Code Review

This pull request introduces contribution guidelines for using AI tools. It adds a new checklist item to the pull request template and a detailed section in Tree-hygiene.md outlining responsibilities for contributors and reviewers when using AI. The changes are clear and well-structured. I have one minor suggestion to correct a grammatical error in the new documentation.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

[stuartmorgan-g]

@jtmcdole Sorry for the initial notification on this not having the right title or description; I hit the wrong series of keys while editing and managed to trigger the submit button. This is ready now.