Add AI Contribution Policy to CONTRIBUTING.md

[silverwind]

Based on my recent experience of both using AI tools and reviewing AI-generated pull requests. Partially based on https://typescript-eslint.io/contributing/ai-policy/.

[wxiaoguang]

By the way, I was thinking about splitting the CONTRIBUTING.md into different files.

There are too many contents mixed together in CONTRIBUTING.md, many of them are for maintainers only, not needed by contributors.

In my mind, some files:

1. CONTRIBUTING.md: only contains the useful information for new contributors, and link to other files
2. docs/guideline-frontend.md
3. docs/guideline-backend.md
4. docs/community-governance.md
5. docs/release-management.md

Otherwise, the all-in-one file which has more than thousand of lines (wrapping....) is difficult to read, and the useful information can be easily ignored (for example: don't rebase+force-push)

[silverwind]

Yeah, makes sense to split up that huge file.

[TheFox0x7]

Overall seems like a good starting point. I'll hold on ACK for the other thread to be resolved.