docs(argo-cd-git-ops): Document how Stackable operator or product updates work

The argo-cd demo currently shows how to deploy Stackable operators and their respective products via ArgoCD and Git. 

It however does not show or explain how to upgrade operators or products, which is a key ingredient of argo and git ops.

This should work pretty much out of the box, but steps should be documented on how to actually upgrade operators or products and what problems could appear.