Doc: Removed obsolete content

Replaced references to WebKit with WebEngine and
updated the information about Qt Cloud services.

Change-Id: I81e04c5310721113feb70c6ac5dd3baaa8830d3a
Task-number:QTBUG-42554
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Nico Vertriest <nico.vertriest@digia.com>