Change: Use own PR template for docs

Docs don't have tests but require a manual entry in the file.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,22 @@
+## What
+
+<!--
+  Describe what changes are being made, e.g. which feature/bug is being
+  developed/fixed in this PR? How did you verify the changes in this PR?
+-->
+
+## Why
+
+<!-- Describe why are these changes necessary? -->
+
+## References
+
+<!-- Add identifier for issue tickets, links to other PRs, etc. -->
+
+## Checklist
+
+<!-- Remove this section if not applicable to your changes -->
+
+- [ ] [Changelog](src/changelog.md) entry
+
+