feat: add Grafana dashboard and ServiceMonitors

[apricote]

Add a default dashboard to visualize the exposed metrics as well as ServiceMonitors and a guide on how to set everything up.

Related to #346

[splattner]

this should also be added to the kustomization.yaml

[apricote]

Hey @splattner, thanks for the suggestion :) I was thinking about that, but wanted to keep the existing kustomization.yaml backwards compatible. The problems with that approach are:

• If we add the ServiceMonitors to the main kustomization.yaml, it will require prometheus-operator to be installed beforehand (or at least its CRDs) and fail for clusters that dont have the operator
• There is no standard way to deploy Grafana dashboards, we could make a ConfigMap out of it though

We did add a new kustomization.yaml that you can use to install the ServiceMonitors: deploy/kubernetes/service-monitor/kustomization.yaml

I also plan on adding this to an upcoming Helm Chart, see #369