GUI in 3d scene deserves a mini tutorial

[chriscamacho]

as per title

a few words about how it works,and what each bit does, would go a long way to making this much more usable, as its a complicated and none obvious demo!

[Calinou]

For future reference, OP is referring to https://github.com/godotengine/godot-demo-projects/tree/master/viewport/gui_in_3d not having a corresponding manual page.