Set up alerts with alertmanager

[d3adb5]

Set up alerts with Prometheus alertmanager. It is likely being deployed already as part of kube-prometheus-stack, but notifications and alerts have not been set up.

For notifications, we can try out different solutions, from Telegram all the way up to PagerDuty (free for up to 5 users). Should the latter be used, let's just make sure nobody gets calls or anything of the sort in the middle of the night. These services are not mission critical, they're not even a product.