Doc: update illustrations on front page

And on other pages. Use a two-column layout on the
Getting Started page to make it more readable in small
browser windows (in Qt Creator, for example).

Change-Id: I362b260305f8f66ae1796911e372858eeb5cc26f
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>