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).

[BlueberryGecko]

Seems there was a CI issue that got fixed in master, so I rebased.

[mhilbrunner]

This looks good to me, and thanks for the rebase! Would you mind taking the chance to convert these images to WebP? We're in the process of moving images over to WebP whenever we update existing ones, see image contribution guidelines for details.