Doc: Update WebView code sample on Qt WebKit landing page

Replace obsolete 'Page' QML type used in the sample code
with 'ScrollView' from Qt Quick Controls.

Task-number: QTBUG-35154
Change-Id: I0a16a6158be028bf8a7975240411841c4e8200ef
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>