Following your link reveals that MongoDB cloud services actually do not provide a complete standalone solution for push notifications. They suggest to setup Firebase Cloud Messaging (FCM) for this and you can configure stitch to trigger a push notification via FCM. You would need to setup firebase (free tier) for that and stitch separately. I do not know if that scales well and is at the time of my answer a big downside of MongoDB services.
Yeah, they just provide an example via Android, but I think using it on client side is very straight forward. The most part is configuring the backends.
I hope that helps.