[Doc] Getting started / introduction documentation page

The Topics section on the main Quick3D doc page should contain a link to a page that gives an overview of the Quick 3D features. (View3D, meshes, materials, cameras, lights, and a few other basic things).

Ideally with code snippets and screenshots.

There is currently no overview of any kind, which makes it quite difficult to get an overview for a new user of what Qt Quick 3D is.

Advanced features, such as custom geometry/texture/effects do not need to be covered on this page, apart from perhaps a mention. Those are to be handled separately in QTBUG-86445. Qt Quick item integration handled separately in QTBUG-85021.