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.

Copyright and Licensing

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

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

[paul-buerkner]

Thanks!

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 4ec8b00 is merged into master:

• ✔️as_draws_array: 158ms -> 158ms [-0.56%, +0.58%]
• ✔️as_draws_df: 84.1ms -> 83.6ms [-1.39%, +0.08%]
• ✔️as_draws_list: 175ms -> 175ms [-0.8%, +0.4%]
• ✔️as_draws_matrix: 24.9ms -> 24.7ms [-1.63%, +0.23%]
• ✔️as_draws_rvars: 132ms -> 133ms [-0.54%, +1.16%]
• ✔️summarise_draws_100_variables: 770ms -> 768ms [-0.56%, +0.15%]
• ✔️summarise_draws_10_variables: 86.6ms -> 86.4ms [-0.58%, +0.14%]
  Further explanation regarding interpretation and methodology can be found in the documentation.