Doc: Updated name of Qt Quick Code Samples page.

-new name is Qt Quick Examples and Tutorials

Task-number: QTBUG-36838
Change-Id: I3516d79bf4d6775f8d21ccb66726ffbbfd4df761
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Doc: Updated main landing page with a new layout and images.

-added image thumbnails
-updated CSS file with recent changes.

Task-number: QTBUG-36411
Change-Id: I4af92292d9b1c139eb8e6f7f03d4fea0ac00a9b0
Reviewed-by: Martin Smith <martin.smith@digia.com>
Merge remote-tracking branch 'origin/stable' into dev

Change-Id: Ia59f5a95ff1bc62d3698e95bcb9bd03caf4578ed