docs(maintainers): define the feature proposal workflow (#7000)

The handbook says substantial features need Product Owner approval
before implementation, but not how a proposal gets there, what counts
as substantial, or what happens when no answer comes. Add the full
workflow: triage within 7 days, a decision brief, four product labels,
a lazy-consensus timeout that only applies to big-but-reversible work,
and definitions for every judgement call (small, big, reversible, new
direction, fits ROADMAP, objection, recorded decision).

The Feature Request form gets three optional fields for the brief, and
the PR template, CONTRIBUTING and triage point at the workflow.

.github/pull_request_template.md

@@ -13,6 +13,9 @@
 
      Private maintainer-preview PRs may instead use:
      N/A — maintainer integration work
+
+     New features: the linked issue carries `product: approved` or
+     `product: maintainer decision` (docs/maintainers/product-and-features.md).
 -->
 
 Fixes #