Allow a book website to have custom pages and navigation items

[MinimizationOrg]

Let's imagine the following situation:

Two years from now, Quarto will have become very popular (which I believe to be likely), and you will then be approached by some publishing houses asking you to publish a Quarto book with them. This book will contain two parts: Guide and Reference, each has its own chapters but there are certainly cross-references between the two parts. How would you do to generate such a book?

To deal with such situation, the best thing to do is perhaps to write the documentation under a book project (the above is just an example, but you should really think about it!). Unfortunately the current book template seems to be too restrictive as it doesn't allow custom pages and navigation items.

A book website usually doesn't contain only the content of the book, but also video lectures, reviews, solutions to the exercises, errata, etc. Currently this doesn't seem to be possible with Quarto.