[Doc] Document the Enhanced Embedded 2D QML Items in 3D in Qt 6

Extend the documentation for embedded 2D Qt Quick Items in 3D to explain the additional features provided in Qt 6.

Items are now rendered without an FBO.
If you want to use an FBO then just enable a layer.
It's possible for anchoring to work now.
Clipping is not possible without a layer.

The quickitems example comes with some documentation, which is good, but this feature would ideally get its own page in the documentation, linked from the front page (Topics section).