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

Follow up to https://github.com/stackabletech/documentation/issues/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

首先阅读后续 issue #282，并检查 commons、secret 和 listener operator 的现有文档。对于每个 operator，记录其角色、创建并保留或移除的资源、依赖项、图表和演示链接。当三个 operator 的复选框全部覆盖后，工作即告完成。