chore(pr-template): add AI usage note

Many PRs now use AI tools. Including this note in PR templates should 1) increase adherence to the AI_POLICY among long-time contributors who may forget to make Assisted-by notes and 2) help inform new contributors about the AI_POLICY.

Author: ericli3690

.github/pull_request_template.md

@@ -1,4 +1,12 @@
 <!--- Please fill the necessary details below -->
+
+<!---
+Uncomment this section if AI tools were used. New contributors may not use AI tools. See:
+https://github.com/ankidroid/Anki-Android/blob/main/AI_POLICY.md
+> [!NOTE]
+> Assisted-by: ___
+--->
+
 ## Purpose / Description
 _Describe the problem or feature and motivation_