Doc: Changed layout to use the empty spaces efficiently

- Added images and text within \raw HTML tables to remove
  the default background and size restrictions.
- Replaced the controls gallery example screenshots with
  the latest ones.
- Added a screenshot of the Qt WebEngine nano browser example
- Fixed a few typos

Change-Id: Ie0d03916db98ef0ad88e6f92e51177a5a8260c9a
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>