text: Title: missing examples in the serial and parallel executor docstrings
  Body: These two sections are missing examples:
  
  https://docs.ploomber.io/en/latest/api/_modules/executors/ploomber.executors.Serial.html
  https://docs.ploomber.io/en/latest/api/_modules/executors/ploomber.executors.Parallel.html
  
  We should add snippets that show how to use the `dotted_path` feature. Same examples as here:
  
  https://docs.ploomber.io/en/latest/api/spec.html#executor