contributors: write down what a PR author can expect from review #6277

Problem

External contributors don't know when, or whether, a review will come. On 2026-10-05 there were 239 open PRs: 51 labeled triage-pending and 59 Stale. On 2026-10-28: 253 open, 48 triage-pending, 21 Stale. CONTRIBUTING.md makes no promise about response time, and it points readers to GitHub Discussions through a README that never links them.

To do

• Write the review expectations in CONTRIBUTING.md: time to first response, what review/triage-pending means, what happens at the stale threshold.
• Either link Discussions from the README or drop the reference to them.

CODEOWNERS is tracked in #5425. Comes from #5465 and #5466.