docs: Add Notification API to docs (#20290)

docs/README.md

@@ -134,6 +134,7 @@ These individual tutorials expand on topics discussed in the guide above.
 * [MenuItem](api/menu-item.md)
 * [net](api/net.md)
 * [netLog](api/net-log.md)
+* [Notification](api/notification.md)
 * [powerMonitor](api/power-monitor.md)
 * [powerSaveBlocker](api/power-save-blocker.md)
 * [protocol](api/protocol.md)