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.

[mhilbrunner]

@BlueberryGecko Much appreciated! Note that the classes/class_vector2.rst file is autogenerated from the source in the main engine repository, as noted at the top of the file, so the changes to that file will get lost. You need to transfer those few changes to a separate PR on the main repository, in this source file: https://github.com/godotengine/godot/blob/master/doc/classes/Vector2.xml

[BlueberryGecko]

@BlueberryGecko Much appreciated! Note that the classes/class_vector2.rst file is autogenerated from the source in the main engine repository, as noted at the top of the file, so the changes to that file will get lost. You need to transfer those few changes to a separate PR on the main repository, in this source file: https://github.com/godotengine/godot/blob/master/doc/classes/Vector2.xml

Ah, that should have been somewhat obvious... I opened the PR (godotengine/godot#112341). I assume the PRs have to be merged somewhat in sync? Let me know if there's anything more I need to do for that.

(I removed the changes to the .rst here and amended the commit. I hope that's correct now.)