New landing page structure

On the landing page there was no generic and basic info on ONAP yet,
 this has been added. A new architecture figure has been created.
ONAP documentation was made available from this main landing page as well.
The side bar of the landing page has also been changed.
 So far the section of the Release note have been made available
 from here, but now only the main doc titles are listed.

Issue-ID: DOC-351

Change-Id: I381c8e232438c51211a8a9303ea3589ebea4d4a4
Signed-off-by: Noemi Wagner <noemi.wagner@nokia.com>
5 files changed