Support notebooks

[davidbrochart]

From demo.md: "In the future I think we could add controls to export the code, copy to the clipboard. Heck, even run it and show the output?" This is going in the direction of notebooks. Although Jupyter notebooks are currently cell based (where each cell can be of code or Markdown type), other notebook formats, like Quarto, are Markdown based with inline fenced code blocks. The Jupyter notebook format might also evolve towards a Markdown-based format in the future.