docs: Update AI usage policy

docs: Update AI usage policy

The previous policy was primarily written from a standpoint
that AI models are not very good and we didn't wanna waste any
time reviewing PRs generated by AI. Now that AI models have become
actually good and their output is just as good as regular contributions,
let's stop requiring the disclosure as its pointless to still have it,
it doesn't really matter anymore whether a patch was written with or without
AI. It's up to the author to make sure they're not wasting our time by
submitting unreviewed, untested code upstream, regardless of whether that
code is written by an AI or not.

The new policy is inspired by https://github.com/lxc/incus/pull/3506, with
various removals to be less adverse to the usage of AI.