Define policy forbidding use of AI code generators

Interesting. Harder line than the LLVM one found at https://llvm.org/docs/DeveloperPolicy.html#ai-generated-cont...

I'm very old man shouting at clouds about this stuff. I don't want to review code the author doesn't understand and I don't want to merge code neither of us understand.

I have just started adding DCO to _all_ of the open source code that I maintain and will be adding text like this on `CONTRIBUTING.md`:

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LLM-Generated Contribution Policy

Color is a library full of complex math and subtle decisions (some of them possibly even wrong). It is extremely important that any issues or pull requests be well understood by the submitter and that, especially for pull requests, the developer can attest to the Developer Certificate of Origin for each pull request (see LICENCE).

If LLM assistance is used in writing pull requests, this must be documented in the commit message and pull request. If there is evidence of LLM assistance without such declaration, the pull request will be declined.

Any contribution (bug, feature request, or pull request) that uses unreviewed LLM output will be rejected.

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I am also adding this to my `SECURITY.md` entries:

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LLM-Generated Security Report Policy

Absolutely no security reports will be accepted that have been generated by LLM agents.

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As it's mostly just me, I'm trying to strike a balance, but my preference is against LLM generated contributions.