add guidelines for AI based contributions

Triggered by #2199, the purpose of this issue to to address whether pygeoapi requires an AI/LLM policy, which will then be addressed by an RFC for PSC vote as well as updates to `CONTRIBUTING.md`.

For context in related projects, here's the RFC and policy from GDAL:

- https://gdal.org/en/stable/development/rfc/rfc111_ai_tool_policy.html
- https://gdal.org/en/stable/community/ai_tool_policy.html

cc @geopython/pygeoapi-team