text: Title: make >>> unselectable in the documentation
  Body: Our docstring examples need to start with `>>>` for pytest to detect them as tests. [Example](https://docs.ploomber.io/en/latest/api/_modules/tasks/ploomber.tasks.PythonCallable.html#ploomber.tasks.PythonCallable):
  
  ```python
  >>> from pathlib import Path
  >>> from ploomber import DAG
  >>> from ploomber.tasks import PythonCallable
  >>> from ploomber.products import File
  >>> from ploomber.executors import Serial
  >>> dag = DAG(executor=Serial(build_in_subprocess=False))
  >>> def my_function(product):
  ...     # create data.csv
  ...     Path(product).touch()
  >>> PythonCallable(my_function, File('data.csv'), dag=dag)
  PythonCallable: my_function -> File('data.csv')
  >>> summary = dag.build()
  ```
  
  However, if users want to select them, they'll also copy the `>>>` characters, and it will break if they put it in the Python interpreter (although IPython is smart enough to ignore them).
  
  I noticed that the [pandas](https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.groupby.html#pandas.DataFrame.groupby) documentation prevents the user from selecting them, along with the `...` characters:
  
  ![image](https://user-images.githubusercontent.com/989250/169142593-4946ad50-f76c-470b-9a35-b7970b6177d3.png)
  
  we should to the same
  