Table of Contents sidebar

I'm just testing the waters here to see if there is any interest in implementing a Table of Contents sidebar, similar to what the Folio Views application displays:

The issue provides no file, test, or entry point to inspect and only asks whether a Table of Contents sidebar is wanted. Start by locating the existing document or HTML rendering and navigation code; the issue needs agreed behavior and scope before completion can be judged.

Written by the indexing model from the issue text.