Add message about 3.15.3 release

Author: josh-wong

src/data/notifications.js

@@ -1,6 +1,17 @@
 // This file contains the notifications data and a function to retrieve it.
 // The notifications are stored in an array of objects, each containing a message in multiple languages and URLs for those messages.
 const notificationsList = [
+  {
+    message: {
+      en: 'Check out the release notes for ScalarDB 3.15.3',
+      ja: 'ScalarDB 3.15.3 のリリースノートをご覧ください'
+    },
+    url: {
+      en: 'releases/release-notes#v3153?utm_source=docs-site&utm_medium=notifications',
+      ja: 'releases/release-notes#v3153?utm_source=docs-site&utm_medium=notifications'
+    },
+    unread: true
+  },
   {
     message: {
       en: 'Discover how to implement vector search capabilities with ScalarDB Cluster',