Clarify Navigation: Chapter Number + Title

[jvaverka]

Breaking out separate request from #48

it would be nice to have the number of a Chapter displayed together with the heading at the top (or indicate the current chapter in some other way) to make navigation easier (maybe this is an issue of Weave).

This would indeed improve the ability to navigate the site since currently the only clue as to which chapter a note belongs to is in the URL, e.g., book.sciml.ai/notes/<chapter number>/.