Documented example(s) for the new way to create custom materials in Qt Quick

Must have a at least one new-style custom material (QSGMaterialRhiShader) example in Qt Quick, with thorough walkthrough (snippets) documentation as well.

The documentation for the QSG classes themselves are in place, but this is not sufficient to allow educating a wide range of users.

 Note: in 5.15 we have one example (quick/scenegraph/twotextureproviders) that shows how to create an application that provides both a QSGMaterialShader and QSGMaterialRhiShader for its custom material. Perhaps extend the documentation for this particular example?