Add Listmonk newsletter configuration example

This provides a template for integrating Listmonk's public API
with the newsletter functionality, including list UUIDs and
basic configuration options.

custom/listmonk-config.example.php

@@ -0,0 +1,16 @@
+<?php
+/**
+ * Listmonk Newsletter Configuration
+ *
+ * Uses Listmonk's public subscription API (no authentication required).
+ * Listmonk handles double opt-in confirmation emails.
+ */
+
+return [
+    'enabled' => true,
+    'url' => 'https://newsletter.example.com', // Your Listmonk URL (no trailing slash)
+    'list_uuids' => [
+        'UUID1',
+        'UUID2'
+    ]
+];