Various doc tweaks

- Fix the example pages titles to match the qdocconf files
- Remove information not as relevant for QtWebEngine as it was for
  QtWebKit
- Fix the QtWebEngineWidgets snippet not to refer to webengine
- Remove the dead link and the reference to Arora from QtWebEngine

Change-Id: Ib3c57c8dd534330b3323c94fb2217374b3e52cf9
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>