consider whether to allow collapsing the TOC

If we need more screen real estate, we could get it by using some of the methods described here: https://quarto.org/docs/authoring/article-layout.html

And by setting `reader-mode: true` in the `_book.yml`

As we get more actual content in the book it'll probably become apparent whether leaving it as it is now will be ok, or if we should try and gain more screen space