Clarify `@icon` SVG Import

## Issue description:

I recently submitted an issue to the main repository, only to learn that what I'd thought was a bug with class icons was *actually* just something I didn't know I was meant to do in the Import options. A contributor directed me to the relevant page in the documentation, which put an immediate end to my problem.

What I noticed, though, is that the documentation could be improved so as to direct one to and from the relevant info on how to make editor icons work properly.

Specifically, the **class @GDScript** page has documentation for annotations, including `@icon()`.

Likewise, the **Editor Icons** page on creating the icons in question is not only a bit hidden away in the **Editor development** section of the docs, but also gives no indication of even the `@icon` annotation’s existence, let alone that this page contains essential info on how to format and import the SVG files you’ll use with the annotation.

## Suggested Improvements

I think, at the very least, the section on the `@icon` annotation should have a link to the **Editor Icons** page, and vice versa. Ideally, the latter should be updated to mention that `.gd` scripts can benefit from custom icons via the annotation—and that the information here applies.

Potentially, I’d suggest considering whether the **Editor development** section might be the wrong place to put the **Editor Icons** page, since editor icons are something most godot users can and should at least know how to work with: not just those who intend to develop the engine code itself. Its current location means most people wouldn’t even think to look for it there.

## URLs to the documentation pages:

https://docs.godotengine.org/en/stable/classes/class_%40gdscript.html#class-gdscript-annotation-icon

https://docs.godotengine.org/en/stable/engine_details/editor/creating_icons.html