All TOC content should be included in the TOC for a book

[jtbayly]

Description

In a book, the left sidebar automatically includes a top-level TOC. I would like the option to show the whole TOC of the book.

Notably, this would also solve the related issue that it is currently impossible to see the TOC of a page on mobile (#5961).

This was already asked as a question, and answered in the negative. This post is just to make an actual feature request. You can read the other discussion here: #5859