Document Z sorting in 2D

[NathanLovato]

Write a page in 2D about Z sorting.

It should explain the default Z order of sprites, properties like showing behind the parent, relative Z values, but also automatic sorting on the Y position. It should probably mention CanvasLayer and maybe link to the viewport documentation, which reset or separate the Z sorting.

Note that Ysort may be removed for Godot 4.0 and replaced by a property on Node2D. See godotengine/godot#42282 for more information.