Allow users to change settings from a notebook view

Currently the Table of Contents sidebar seems to be collapsed by default and must be opened either from `View > Table of contents` or shift-command-k`.

It would be useful the show / hide setting could be set by default via a settings file.

Ideally, the user should also be able to change the default, but I note that the notebook UI does not seem to expose the `Settings > Settings Editor` option that JupyterLab provides?

Start by reproducing the notebook UI behavior: check the Table of Contents under View and the missing Settings > Settings Editor entry. Trace how notebook settings are currently loaded and saved, then define the supported default and verify that the sidebar state persists and can be changed through the available settings UI.

Written by the indexing model from the issue text.