Switch tutorial rendering to Jupyter Book (or something similar)

The quick and simple notebook structure and rendering in #2 gets the job done, but requires manually creating an index notebook, and will struggle with things like cross-referencing later. Eventually we should switch to a tool that can handle generating the whole book and has these extra features.

Options include:
* [Jupyter Book](https://jupyterbook.org/en/stable/intro.html), which [supports Jupytext format sections](https://jupyterbook.org/en/stable/file-types/jupytext.html)
* [nbdev](https://nbdev.fast.ai/) New tool from Fast.ai that is specifically designed to generate software packages _and_ documentation from notebooks. This might be overkill, although I'm intrigued. Under the hood, it uses [Quarto](https://quarto.org/) to generate pages.

Start by reviewing the notebook structure and rendering introduced in #2. Compare Jupyter Book, nbdev, and Quarto against the stated needs, including Jupytext support and cross-referencing. Done means selecting and adopting a tool that generates the whole tutorial without a manually created index notebook.

Written by the indexing model from the issue text.