Add tutorial for spline-based movement of actors (for cars, people etc.)

[j9liu]

There's a neat tutorial that shares how to animate models along a spline. This could be used to add cars, people, etc. moving along specified paths, adding more realism in a Cesium environment.

The tutorial itself isn't specific to Cesium, but it would be nice to adapt it into a Cesium tutorial that specifically places these elements on top of 3D Tiles.