GDK restyling: lowercasing tag names
GDK restyling: copy resources (in particular the groovy icon and the stylesheet)
GDK restyling: more tag lowercasing
GDK restyling: use the center class for all bodies
GDK restyling: changes to the left frame pages, and update of the main content by removing some unneeded nav sections without pages
GDK restyling: adapt the navigation bar
GDK restyling: styling of the classes template
Merge pull request #370 from glaforge/gdk-restyled

GDK documentation restyling