Improve CONTRIBUTING doc

[philrz]

What's Changing

The guidelines for contributions is enhanced with additional clarity about the level of human interaction we ask of community members before we'll review a Pull Request.

Why

Like many open source projects, we've had a recent flurry of PRs that read as if they were created wholly via AI. These often don't provide a proper fix/enhancement and/or don't fit well within existing design/architecture and so we've often closed them without comment. By clarifying this in the guidelines we'll hopefully dissuade these kinds of contributions and/or give us something to point to in a comment when closing such PRs should they continue to appear.

Details

If this gets merged, I'd plan to paste a comment like this in future such PRs as I close them (feel free to propose edits):

Hi! We're closing this PR because it doesn't appear to have the prior issue discussion described in our CONTRIBUTING.md. Before writing code, we ask that contributors comment on the relevant issue describing their intended approach and hear back from a maintainer first.

If you're still interested in this change, please comment on the issue with your thinking and we'll go from there. If you feel we've misread this PR, feel free to reopen and let us know.

I'd also go back to some of the recent PRs we've closed and paste a similar comment pointing at the updated guidance.