Clean Up Manim's API

[JasonGrace2282]

Currently, it's difficult to tell which methods are internal to Manim, and which are designed to be used by an end user. Once the API of Manim is solidified, it's easier to make more drastic changes without having to worry about breaking user code, because all you have to do is keep the public interface stable (ish).

Renaming methods to start with an _ to avoid them being documented is not a good solution, as the documentation should be for both developers and end users.