docs: PR template (#2580)

.github/pull_request_template.md

+## Description
+
+<!-- Include relevant issues or PRs here, describe what changed and why -->
+
+
+## Suggested changelog entry:
+
+<!-- fill in the below block with the expected RestructuredText entry (delete if no entry needed) -->
+
+```rst
+
+```
+
+<!-- If the upgrade guide needs updating, note that here too -->