✨ docs: add AI usage disclosure policy

[vmaerten]

Summary

• Add an AI Usage Policy section to the contributing guide, requiring contributors to disclose AI-assisted code (similar to oxc)
• Create a root CONTRIBUTING.md with the policy and a link to the full guide on taskfile.dev
• Add the same policy section to website/src/docs/contributing.md (before "Before you start")

[trulede]

I would almost suggest that code should predominantly be written by contributors.

This AI generated policy is also quite nice (irony is noted):

"Code should be written by contributors, though the use of AI tools for assistance—such as refactoring or checking changes—is encouraged. Please note, however, that pull requests (PRs) consisting primarily of AI-generated code are likely to be closed."