Doc: Added detailed description for WebView

Moved the description from the overview page
to the WebView type documentation. Also added an
image corresponding to the snippet used.

Task-number: QTBUG-36179
Change-Id: Iaf4207a36f774d93af9a881eae2162d44837eeee
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>