add AI contribution policy

[florence-bockting]

Summary

This PR adds a reference to the Stan AI Contribution Policy (see Stan-wiki: https://github.com/stan-dev/stan/wiki/AI-Contribution-Policy) to the CONTRIBUTING.md so contributors are aware of it before submitting PRs.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.75%. Comparing base (85db294) to head (892c9c8).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #361   +/-   ##
=======================================
  Coverage   92.75%   92.75%           
=======================================
  Files          31       31           
  Lines        2983     2983           
=======================================
  Hits         2767     2767           
  Misses        216      216           

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[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 892c9c8 is merged into master:

• ✔️loo_function: 1.77s -> 1.77s [-0.43%, +0.52%]
• 🚀loo_matrix: 1.32s -> 1.31s [-0.84%, -0.06%]
  Further explanation regarding interpretation and methodology can be found in the documentation.