Doc: Fix issues related to example docs

    - Add documentation for Quick Nano Browser
    - Add all examples to the correct group
    - Add instructions to run the examples
    - Qt WebEngine Widgets index page: Add examples section

Task-number: QTBUG-42668
Change-Id: Ie60f8708336b9c147d0c22afb555fe87287deddc
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>