Fix and document how to use `stage_scenes.py` (e.g. example)

I can't figure out what `stage_scenes` does or how to use it - I'd appreciate some documentation or at least a one-line example. :)

I'm trying to package manim and I'm trying to figure out which files should be runnable. `manim.py` and `stage_scenes.py` seem to be the only, right?