text: Title: update documents to reflect new default format
  Body: initially, we were using the `py:light` format as the default one for scripts:
  
  ```python
  # +
  x = 1
  
  # +
  y = 2
  ```
  
  However, we're now migrating to the percent format to be the default, since VSCode, PyCharm, and others support it:
  
  ```python
  # %%
  x = 1
  
  # %%
  y = 2
  ```
  
  Some sections in our documentation need updates:
  
  https://docs.ploomber.io/en/latest/user-guide/jupyter.html
  https://docs.ploomber.io/en/latest/get-started/basic-concepts.html
  
  I found these two but there may be others.
  