Create a (mobile friendly) double TOC layout

[Rapptz]

This is a blocker for basically everything.

Right now the current TOC is on the left side of the page, this is called the local TOC. However, Sphinx also has something called global TOC. It would be ideal to have the global TOC on the left and the local TOC on the right in the desktop view.

Certain projects and themes already do this:

• https://docs.godotengine.org/en/stable/index.html (RTD theme, no local TOC on right)
• https://sphinx-themes.org/sample-sites/furo/ (furo theme, local TOC on right)

The global TOC can be accessed from a template using the toctree callable and the local TOC is using the toc variable.

It needs to be mobile friendly of course. Once this is done then it should be possible to do everything else.