chore: add pull request template

Author: rasros

.github/pull_request_template.md

@@ -0,0 +1,12 @@
+## What changed
+
+<!-- Bullet list of changes. Simple sentences, no special formatting. -->
+-
+
+## Why
+
+<!-- Simple sentences. No headers, bullets, or code blocks unless load-bearing. -->
+
+## Testing
+
+<!-- Simple sentences. How you verified it. -->