Introduction of an AI-policy for stan-dev repos

[florence-bockting]

Description

This pull request introduces an initial proposal of an AI contribution policy that addresses the use of Large Language Models (LLMs) and generative AI tools in code contributions. The policy is designed to be adopted across Stan GitHub packages in the future. Given the growing use of AI-assisted tools, the goal is to provide clear guidance for our contributors.

This PR includes two documents:

• an AI policy document (ai_policy.md)
• a PR template document that builds upon the AI policy document

We invite the community to review the drafts, share ideas and feedback, and provide input before we test the documents in a selected repository. Thank you.

An initial draft of the AI-policy was generated using Google Gemini.

[bob-carpenter]

Thanks for submitting. I have a few questions on the legalese, some grammatical corrections, and some policy questions.

[bob-carpenter]

This PR is not ready for approval until [date] is replaced with an actual date.

[WardBrian]

The rendered policy can be viewed here: https://github.com/florence-bockting/design-docs/blob/ai-policy/ai_policy/AI_policy.md

I'm curious about the DCO requirement. We haven't previously required it, since the Github Terms of Service contain fairly similar language. The DCO goes a bit further, nearly to the point that one could interpret it as prohibiting any AI-written code until the copyright law about it settles down...

[bob-carpenter]

I'm OK with this now, so I'll submit my approval so it's not blocking anything. But I think we need something like SGB approval to launch a new policy.