Add example rule

Migrated from https://github.com/couchbaselabs/observability/issues/8. 

Provide an example of a rule we can trigger with a custom annotation to show how we would link to a URL explaining how to resolve it.

Existing Loki examples: https://www.github.com/couchbaselabs/observability/tree/main/microlith%2Fprometheus%2Falerting%2Floki-alerts.yaml

We can likely set up templates as well to make it easier.

https://prometheus.io/docs/prometheus/latest/configuration/alerting_rules/

Potential good example to use is the auto-failover and single node warnings.