Better documentation for viewports and split screen and world_2d implementation

[ianmckenna87]

Subviewports let you assign a world_3d but not a world_2d in the editor. While trying to implement split screen multiplayer in Godot 4, which I imagine isn't too uncommon, I've had to rely on outdated third party tutorials for Godot 3. There isn't anything in the docs covering this, and the lack of world_2d options for viewports in the editor don't provide much of a hint.

I don't think dedicated node for handling split screen would be such a terrible idea either