Improve the integration of QtWebsockets into general Qt docs

Improve the integration of QtWebsockets into general Qt docs

1.) The websocket qml types page is now hooked into Qt's QML Type by Module
page.

2.) Correct incorrectly documented import statement for Qt.WebSockets
module in QML.

3.) Add missing \since tags (see QTBUG-36775).

Change-Id: I1321db70215fcd16cfbd77dd71a459244241d8a3
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>