Add documentation about the order of execution for node callback functions

**Your Godot version:** 3.6 / 4.2

**Issue description:**

I'm implementing a state machine and I'm curious about when `_unhandled_input()` happens (related to callbacks like `_process()` and `_physics_process()`).

Except digging into the source code, the only source I can find is [this blog post](https://godotengine.org/article/agile-input-processing-is-here-for-smoother-gameplay/).

I think diagram and simple descriptions like [this](https://docs.unity3d.com/Manual/ExecutionOrder.html) benefits a lot.