Less intrusive Table of Contents

[Grinnz]

On larger devices, the table of contents could be displayed to the side so as not to take up huge vertical space in the document, and ideally remain visible while scrolling the document.

To complicate things, this should be as simple as possible (hopefully using built in Bootstrap mechanisms?), and not use any javascript except for what's needed to keep it responsive: on smaller devices, it should revert to a vertical ToC.

[Grinnz]

This also needs to coexist with the option to expand the document to full-width, which implies it should be collapsible.