Doc: Add Qt Multimedia platform notes for Windows

Add a page with details about Qt Multimedia backends on Windows, and
link to it from the module index page.

Initially, cover the status of camera backend in WMF and DirectShow
plugins.

Task-number: QTBUG-29552
Change-Id: I53e5901e2d621c4bbc96a61d5dbb249b4debb8d4
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>