The new documentation style for publishing docs under qt.io
requires dedicated div classes for pages that use a three-
column layout.

The new divs enable the columns to stack and react to changes
in the window width.

Task-number: QTBUG-42086
Change-Id: Ic99a768e82e3eefaacff85458cb1cdf10b852318
Reviewed-by: Martin Smith <martin.smith@digia.com>