Document the shadergen tool

We need documentation about what the shadergen tool is and how to use it, with examples etc.

 
Important topics should included how to handle properties that changes during runtime, naturally you can't expect that to work as that's a variant that was unknown at build time. If a runtime change is needed it can be solved by creating a new component and modify the property in the same way it would at runtime, e.g.:

// MaterialComponentA.qml
PrincipledMaterial {
    baseColor: "blue"
}

If we want to change the color input for CompA at runtime to use a map instead, we'll need to define
a version of that material somewhere in our project.

// MaterialComponentAmap.qml
MaterialComponentA {
    baseColorMap: url
}