Add LLM usage checkbox to PR template (#2294)

Author: yeongbin05

.github/PULL_REQUEST_TEMPLATE.md

@@ -10,3 +10,6 @@ Fixes # (issue)
 
 - [ ] I have added the relevant tests for this change.
 - [ ] I have added an item to the Pending section of ``docs/changes.rst``.
+
+#### AI/LLM Usage
+- [ ] This PR includes code generated with the help of an AI/LLM