Doc: Describe functions for inspecting web content in Features topic

Add a short description and links to the new QWebEnginePage functions
and WebEngineView QML properties.

Task-number: QTBUG-66780
Change-Id: I205d1d922caac1c82a11054055ec44924942249d
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>