Check appropriate format and nesting

The migrate tool for Jupyter Book v0.11 automatically set the ["format: " entry](https://github.com/jni/using-python-for-science/blob/08992d40aabb143c8cd53e7dcad81af06328015c/using-python-for-science/_toc.yml#L3) in our _toc.yml to "jb-article". The [documentation](https://jupyterbook.org/customize/toc.html) suggests the format "jb-book", which I did, and structuring the text into Parts, Chapters, and Sections, which I didn't — our highest level of grouping is chapters. However we do have certain chapter groupings (Get it working, Get it right, Get it fast, Ship it) that could be made into parts, so we could consider grouping other chapters similarly.