Demonstrate in examples/tests that custom multiview-capable QSGMaterials and ShaderEffects can be created by the end users as planned

There is nothing in Qt Quick that ensures that the built-in materials (for Rectangle, Text, etc.) do not regress when it comes to multiview support. Similarly, there are no examples or tests in Qt Quick that demonstrate or exercise creating custom QSGMaterials or ShaderEffects in a manner that they are both compatible with regular and multiview rendering setups. Therefore, it is important that Qt Quick 3D (XR) exercises and demos this path, in manual tests and examples.

Check if the documentation needs improvements. Thinking of https://doc-snapshots.qt.io/qt6-dev/qsgmaterial.html#viewCount and the qsb and CMake documentation pages (VIEW_COUNT, MULTIVIEW).