Document the breaking-change advisory block in the PR template

Explains the ```breaking-change block a breaking-change PR can carry —
severity, action (warn/block), expect, and before_update steps — with a
copy-paste example kept inside an HTML comment so it stays inert until an
author fills it in.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TFBuZEp142Ei2PAfWqbMXT

Author: michelroegl-brunner