Docs index is unreadable on mobile

[spotlesstofu]

First, thanks for closing #754!

The documentation index has overlapping sections if you open it on a small screen.

Screenshot below.

[ecederstrand]

Eww, that's ugly. The "minimal" theme default page handles this much better: https://orderedlist.com/minimal/

I added some custom css in https://github.com/ecederstrand/exchangelib/blob/master/docs/assets/css/style.scss to get a wider righ-hand column because it wasn't wide enough for the boxes with example code. I may have broken the layout on small screens along the way.

[ecederstrand]

This should be fixed after a series of commits ending in 5e9ca04