docs: add PR template with useful reminders :)

.github/pull_request_template.md

@@ -0,0 +1,12 @@
+<!--
+If you are a core dev:
+
+Don't forget to set up the fields:
+- assign yourself
+- set priority to same as original issue
+- add PR to sprint
+- if Draft -> "in progress"
+- if final PR -> "needs review"
+
+If you're an external contributor, you can ignore this text.
+-->