Python: Duplicate code is doc strings

[TaoChenOSU]

We have a lot of duplicated code in doc strings, and we should spend some time cleaning them. For example:

1. https://github.com/microsoft/agent-framework/blob/main/python/packages/core/agent_framework/_workflows/_workflow_builder.py#L122
2. https://github.com/microsoft/agent-framework/blob/main/python/packages/core/agent_framework/_workflows/_workflow_builder.py#L475

We should really have the sample code in a single place, and it should probably be in the method doc string, not the class doc string.