Editable Children is undocumented

[Theome]

Your Godot version:

4.4

Issue description:

Editable Children seems to be a much-used and powerful feature, but it's not documented. The feature is mentioned in the documentation of Node's method is_editable_instance(node: Node), but it is not explained what it does.

URL to the documentation page (if already existing):

https://docs.godotengine.org/en/stable/classes/class_node.html#class-node-method-is-editable-instance

[tetrapod00]

The right place for this is probably a new short page in Core Features, or a new header section in Nodes and scene instances. Note that this section should probably be reorganized a bit but we're waiting until the RTD redirect limitation has been improved somehow.