[meta] Update contributing guidelines docs/template regarding AI-generated/vibe coded contribs

From the GitHub side of things (manually imported by yours truly):

Let's not talk about the recent dumpster fire at @hackclub Slack's meta channel (context, context, context) and this merge request in question, but part from it there's been a growing number of open-source maintainers banning AI-generated/vibe-coded code and non-code contributions lately.

Although I might be guilty of this occasionally in the past (debugging hell still happens and reviewing the code do before git pushing to production), I think we need clarity regarding this in the contributing guidelines, especially around the legalese of it.

Going to check notes from other projects regarding this soon and will link them here for prosperity.