docs: def start_of_new_markdown_cell(self, line):
          """Does this line starts a new markdown cell?
          Then, return the cell marker"""
          for empty_markdown_cell in ['""', "''"]:
              if line == empty_markdown_cell:
                  return empty_markdown_cell
  
          for triple_quote in ['"""', "'''"]:
              if line.startswith(triple_quote):
                  return triple_quote
  
          if self.twenty_hash.match(line):
              return line
  
          return None