Skeleton 2D docs do not match Godot 4

**Your Godot version:**

v4.0.beta.custom_build [4935493f7]

**Issue description:**

I think Godot 4 removed 2D IK chains, and replaced it with the Bone2D system.

There isn't really any documentation for how to use the Bone2D system, or how to use it to do IK.
If anyone knows how to use the new system, could you update the docs or explain here how it works?
Thanks!

**URL to the documentation page:**

This page is no longer relevant:
https://docs.godotengine.org/en/latest/tutorials/animation/cutout_animation.html

The information on this page is no longer correct:
https://docs.godotengine.org/en/latest/tutorials/animation/2d_skeletons.html