Improve default template of DocumentationViewer
Improve default template of DocumentationViewer

class "container" was missing, so a bunch of CSS wasn't being applied.
The layout of the default docsviewer checkout now looks more like the
one on doc.silverstripe.org.

camfindlay/silverstripe-docsviewer,dhensby/silverstripe-docsviewer,silverstripe/silverstripe-docsviewer,silverstripe/silverstripe-docsviewer,dhensby/silverstripe-docsviewer,camfindlay/silverstripe-docsviewer