Add AI contribution policy

[florence-bockting]

Submission Checklist

• [ ] Run unit tests
• Declare copyright holder and agree to license (see below)

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.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Aalto university

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.23%. Comparing base (a8adbb1) to head (2f90c03).
⚠️ Report is 157 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1189      +/-   ##
==========================================
+ Coverage   87.97%   91.23%   +3.25%     
==========================================
  Files          14       15       +1     
  Lines        5937     6125     +188     
==========================================
+ Hits         5223     5588     +365     
+ Misses        714      537     -177     

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