Show handedness in Vector2's angle-related images

[BlueberryGecko]

Imo the handedness of angles in the documentation images for methods such as Vector2.angle_to should be explicitly shown.

My hope is that this adds visual clarification/intuition that:

1. these three methods could return negative angles (counterclockwise)
2. the methods are not symmetric (e.g. a.angle_to(b) != b.angle_to(a))
3. Godot's angles increase in clockwise direction (pretty standard in computer graphics, but easy to confuse beginners).