Docs: Increase the visibility of our genAI disclosure requirement (#60649)
Currently, we [already
mention](https://github.com/JuliaLang/julia/blob/master/CONTRIBUTING.md#contributor-checklist)
this in `CONTRIBUTING.md`. But that's clearly not sufficient (because we
keep getting genAI PRs without disclosure).
So this PR tries to make our disclosure requirement easier to notice:
1. Put a note in the README.
2. Put another note at the top of CONTRIBUTING, with the purple
"important" formatting, to try to increase visibility.[^1]
3. Add two notes to AGENTS.md.[^2]
- Tell the AI tool to add itself as a Git co-author on commits.
- Tell the AI tool to disclose AI use when opening the PR.
[^1]: Currently, we only mention the disclosure requirement further down
in CONTRIBUTING. Maybe people aren't scrolling that far down, so perhaps
it'll help to have the note near the top of CONTRIBUTING.
[^2]: Suggested in
https://github.com/JuliaLang/julia/pull/59484#issuecomment-3723935315