Add an explanation of categories for the Visual Profiler

**Your Godot version: 4.5.1

**Issue description: The Visual Profiler documentation doesn't explain what the various categories shown in the profiler are, and some of them are not self-explanatory (e.g. "Prepare 3D Scene" or "Setup 3D Scene"). It would be fantastic to have a table explaining what exactly each of them encompass.

For example, I have this issue in my current project where "Setup 3D Scene" which normally takes 0.4ms spikes at 3.5ms every few frames on the GPU. Not understanding what processes are part of this category (as well as the CPU/GPU breakdown of those processes) means I'm a bit stuck debugging this behavior. There was a post on the Godot forum recently asking what is in this category and it never received an answer: https://forum.godotengine.org/t/visual-profiler-optimization-what-exactly-is-setup-3d-scene-doing/117099. In my opinion, this lack of documentation can severely impact the utility of the visual profiler.

**URL to the documentation page (if already existing): https://docs.godotengine.org/en/stable/tutorials/scripting/debug/debugger_panel.html#visual-profiler