Add visuals to explain Vector methods

**Your Godot version:** v4.2.1

**Issue description:**
Godot has some methods in the Vector API with uncommon names: e.g. `Slide`, `Bounce`, and `Reflect`. I have had to explain these methods to both new and experienced developers.

There have already been discussions and PRs about the need to clarify these methods in the docs:
#8286
#7664
https://github.com/godotengine/godot/pull/68392
https://github.com/godotengine/godot/pull/83691
https://github.com/godotengine/godot/issues/11678

There's enough confusion around these methods that adding graphics to the Vector docs is probably justified. It would be nice to point beginners to the docs whenever someone asks about these methods rather than pull out the MS Paint diagram :)

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_vector2.html
https://docs.godotengine.org/en/stable/classes/class_vector3.html