Nodes: Remove "Use Nodes" in Shader Editor for World

Part of https://projects.blender.org/blender/blender/pulls/141278

Blend files compatibility:
If a World exists and "Use Nodes" is disabled, we add new nodes to the
existing node tree (or create one if it doesn't) that emulates the
behavior of a world without a node tree. This ensures backward and
forward compatibility.

Python API compatibility:
- `world.use_nodes` was removed from Python API => **Breaking change**
- `world.color` is still being used by Workbench, so it stays there,
although it has no effect anymore when using Cycles or EEVEE.

Python API changes:
Creating a World using `bpy.data.worlds.new()` now creates a World with
 an empty (embedded) node tree. This was necessary to enable Python
scripts to add nodes without having to create a node tree (which is
currently not possible, because World node trees are embedded).

Pull Request: https://projects.blender.org/blender/blender/pulls/142342

tools/utils_doc/code_layout_diagram.py

@@ -934,8 +934,10 @@ def render_output(scene, bounds, filepath):
     scene.render.filepath = filepath
 
     world = bpy.data.worlds.new(name_gen)
-    world.color = 1.0, 1.0, 1.0
-    world.use_nodes = False
+    output = world.node_tree.nodes.new("ShaderNodeOutputWorld")
+    background = world.node_tree.nodes.new("ShaderNodeBackground")
+    world.node_tree.links.new(output.outputs["Surface"], background.outputs["Surface"])
+    background.inputs["Color"].default_value = 1.0, 1.0, 1.0, 1.0
     scene.world = world
 
     # Some space around the edges.