Unify and expand change-note guidance

Move and consolidate change-note validation guidance into docs/change-notes.md, removing the separate query- and library-pack instruction files. Add a generic .github/instructions/change-notes.instructions.md (applyTo: "**/change-notes/*.md") that tells reviewers to ensure change notes follow the docs and to leave clear review comments and code suggestions when they do not.

Author: mario-campos

.github/instructions/change-notes.instructions.md

@@ -0,0 +1,8 @@
+---
+applyTo: "**/change-notes/*.md"
+---
+
+Ensure that change notes follow the project's guidelines as described in `docs/change-notes.md`. If the change note does not satisfy the aforementioned guidelines:
+
+1. Leave a review comment that clearly explains the problem with the change note and how it can be improved.
+2. Offer a code suggestion that would fix the problem.