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.

Checklist

• Copyright holder: Aalto university

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the 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/)

• [ ] the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• [ ] the code is written in idiomatic C++ and changes are documented in the doxygen

• [ ] the new changes are tested