We need a QRHI porting guide

As the rendering pipeline is changed with introduction of QRHI. We need a document that describes the common use-cases of customers using Qt Quick 3D and thus educate with functional examples what they need to do on their side when they switch to Qt6.

Use-cases to be described

• no shader code and no direct OpenGL usage => everything works out of the box
• Usage of ShaderEffect or QSGMaterialShader => slight changes needed
• Usage of OpenGL by the app, for example connecting to signals => Forcing QRHI to use OpenGL backend 
• Usage of QQuickFramebufferObject or QQuickWidget => similar to direct usage of OpenGL
• Usage of QQuickRenderControl need to use new QQuickWindow APIs.