Doc: Added initial set of overviews and image

-added components into their groups
-made group overviews
-added links to overviews from landing pages
-added a place holder image

Change-Id: I099de1f8ef3a89cd5f61220874fc69f781704799
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>