Add GenAI Policy

[jfrost-mo]

Adds a policy on the use of Generative AI, both to the documentation, and to the checklist in the pull request template. I've included the rational in Issue #623, so that is worth a read when reviewing.

Fixes #623

Contribution checklist

Aim to have all relevant checks ticked off before merging. See the developer's guide for more detail.

• [x] Documentation has been updated to reflect change.
• [ ] New code has tests, and affected old tests have been updated.
• [x] All tests and CI checks pass.
• [x] Added an entry to the top of docs/source/changelog.rst
• [ ] Conda lock files have been updated if dependencies changed.
• [x] Marked the PR as ready to review.

[github-actions[bot]]

[jfrost-mo]

TODO:

• [x] Add policy to dev guide, with example of citing.