Add page demonstrating the various ways to enable/disable scenes and/or nodes

**Your Godot version:**
3.3.2

**Issue description:**
Basically, make a page out of this answer on the Q&A site:
https://godotengine.org/qa/49696/how-to-disable-enable-a-node?show=49754#a49754
Expand it and fill in the details and caveats of course.

Maybe put it next to this page:
https://docs.godotengine.org/en/latest/tutorials/scripting/nodes_and_scene_instances.html