Operator overview pages for internal operators (commons, secret, listener)

[fhennig]

Follow up to #282

Explain for every operator:

• which roles does it have and what do they mean?
• Which resources does it create:
  • i.e. statefulsets per role
  • configmap for discovery
• also a diagram
• which resources does it tear down again?
  • for example, ConfigMaps and PVCs are staying
• what are the dependencies, what other operators does it connect to?
• link to demos

Operators:

• commons
• secret
• listener