docs: add pull request template

Adds PR template matching the format documented in CONTRIBUTING.md.
Completes one of two remaining GitHub Community Standards items.

Fixes #1553

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,13 @@
+## What
+<!-- 1-2 sentences describing WHAT changed -->
+
+## Why
+<!-- 1-2 sentences explaining WHY this change is needed -->
+Fixes #
+
+## How
+<!-- 2-3 bullets listing HOW you implemented it -->
+-
+
+## Testing
+<!-- 1-2 sentences on how you tested this -->