Add label docs-update-needed for PRs that modify app.example.ini (#31810)

To help #31536.

Or it's easy to forget to update https://gitea.com/gitea/docs when
modifying `app.example.ini`.

.github/labeler.yml

@@ -75,3 +75,8 @@ modifies/js:
       - any-glob-to-any-file:
           - "**/*.js"
           - "**/*.vue"
+
+docs-update-needed:
+  - changed-files:
+      - any-glob-to-any-file:
+          - "custom/conf/app.example.ini"