"Editor icons" page is outdated

[Meorge]

Your Godot version: 4.3

Issue description: It sounds like the explanation of how to optimize editor icons given on the "Editor icons" page of the documentation is now outdated, and that the pre-commit hook should take care of optimization properly.

I found this after using svgcleaner as suggested on the "Editor icons" page to optimize an icon, and having it be rejected by the testing system for not matching the output of svgo (as far as I understand).

URL to the documentation page: https://docs.godotengine.org/en/stable/contributing/development/editor/creating_icons.html#icon-optimization