Update PR template with docs prompt (gatsbyjs#19949)

* chore: update PR template with docs prompt

* chore: improve wording of PR template

Author: Marcy Sutton

.github/PULL_REQUEST_TEMPLATE.md

@@ -12,6 +12,18 @@
 
 <!-- Write a brief description of the changes introduced by this PR -->
 
+### Documentation
+
+<!--
+  Where is this feature or API documented?
+
+  - If docs exist:
+    - Update any references, if relevant. This includes Guides and Gatsby Internals docs.
+  - If no docs exist:
+    - Create a stub for documentation including bullet points for how to use the feature, code snippets (including from happy path tests), etc.
+  - Tag @gatsbyjs/learning for review, pairing, polishing of the documentation
+-->
+
 ## Related Issues
 
 <!--