Add audio distance and attenuation visualization to AudioStreamPlayer2D

Idea shameless stolen from here: https://www.reddit.com/r/godot/comments/ewn1wv/audiostreamplayer2d_visualization/

This commit adds visual representations of the AudioStreamPlayer2D's distance and attenuation (note that the attenuation drawing is disabled by default, as it can make the scene too noisy if there are multiple players):

As this adds documentation to the new members, the leftovers of AnimationTreePlayer's docs have been removed automatically in process.