add guidelines for AI based contributions

[tomkralidis]

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