Allow users to navigate between book pages from a sidebar using the book's Table of Contents

[choldgraf]

When a user browses a page that is part of a multi-page book, they want to be able to navigate between pages in an intuitive way, defined by the book's content flow. The way this is specified in MyST/Jupyter Book is via the Table of Contents, and this is what makes up the sidebar in a MyST site. However, no such view across all pages exists in the JupyterLab extension.

User story

As an author / reader, I'd like to access a table of contents that allows my to navigate between the table of contents of a book without leaving JupyterLab.