Consolidate contribution guidelines for group-external contributors

[peanutfun]

We recently had several group-external contributors using LLMs to improve parts of our code base. Therefore, we want to revise the contribution guidelines:

• Add two ticks to the PR checklist: Users should either check that they did not use AI or that they used AI and carefully checked the model output before submitting the code in the PR.
• Split AUTHORS.md into authors and contributors. Authorship requires a "substantial contribution" to the code base and whether a contribution is substantial will be decided by the core developers.
• Reflect these changes in the CONTRIBUTING.md

[peanutfun]

Maybe adopt an AI/LLM policy? https://github.com/pypa/pip/blob/main/AI_POLICY.md