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.

In fact, we already have one example (quick/scenegraph/twotextureproviders) that shows how to create an application that provides both a QSGMaterialShader and QSGMaterialRhiShader for its custom material.

So an option is to extend the documentation for that particular example.