Docs: Add note in 'material' component or FAQ about texturing models

Common questions we should probably cover somewhere:

1. The material component doesn't work on loaded models, except for OBJ models that do not already have an MTL (material) file.
2. It's usually better to texture in DCC software (e.g. Blender). (Link to tutorial?)
3. If you do need to texture or re-color something on-the-fly, use three.js APIs like this:

el.addEventListener('model-loaded', function(event) {
  event.details.model.traverse(function(object) {
    if (object.isMesh) {
      object.material.foo = bar;
    }
  });
});

For documentation on THREE.Material and its subclasses, see: https://threejs.org/docs/#api/materials/Material

Locate the material component documentation or FAQ entry as the possible destination. Add the requested guidance about loaded models, DCC texturing, runtime three.js material changes, and the THREE.Material reference; done means these common questions are documented clearly in one suitable place.