Shader Nodes: add Python API for inlined shader nodes

This makes the shader node inlining from #141936 available to external renderers
which use the Python API. Existing external renderer add-ons need to be updated
to get the inlined node tree from a material like below instead of using the
original node tree of the material directly.

The main contribution are these three methods: `Material.inline_shader_nodes()`,
`Light.inline_shader_nodes()` and `World.inline_shader_nodes()`.

In theory, there could be an inlining API for node trees more generally, but
some aspects of the inlining are specific to shader nodes currently. For example
the detection of output nodes and implicit input handling. Furthermore, having
the method on e.g. `Material` instead of on the node tree might be more future
proof for the case when we want to store input properties of the material on the
`Material` which are then passed into the shader node tree.

Example from API docs:
```python
import bpy

# The materials should be retrieved from the evaluated object to make sure that
# e.g. edits of Geometry Nodes are applied.
depsgraph = bpy.context.view_layer.depsgraph
ob = bpy.context.active_object
ob_eval = depsgraph.id_eval_get(ob)
material_eval = ob_eval.material_slots[0].material

# Compute the inlined shader nodes.
# Important: Do not loose the reference to this object while accessing the inlined
#   node tree. Otherwise there will be a crash due to a dangling pointer.
inline_shader_nodes = material_eval.inline_shader_nodes()

# Get the actual inlined `bpy.types.NodeTree`.
tree = inline_shader_nodes.node_tree

for node in tree.nodes:
    print(node.name)
```

Pull Request: https://projects.blender.org/blender/blender/pulls/145811

doc/python_api/sphinx_doc_gen.py

@@ -2189,6 +2189,24 @@ def write_rst_geometry_set(basepath):
     EXAMPLE_SET_USED.add("bpy.types.GeometrySet")
 
 
+def write_rst_inline_shader_nodes(basepath):
+    """
+    Write the RST files for ``bpy.types.InlineShaderNodes``.
+    """
+    if 'bpy.types.InlineShaderNodes' in EXCLUDE_MODULES:
+        return
+
+    # Write the index.
+    filepath = os.path.join(basepath, "bpy.types.InlineShaderNodes.rst")
+    with open(filepath, "w", encoding="utf-8") as fh:
+        fw = fh.write
+        fw(title_string("InlineShaderNodes", "="))
+        write_example_ref("", fw, "bpy.types.InlineShaderNodes")
+        pyclass2sphinx(fw, "bpy.types", "InlineShaderNodes", bpy.types.InlineShaderNodes, False)
+
+    EXAMPLE_SET_USED.add("bpy.types.InlineShaderNodes")
+
+
 def write_rst_msgbus(basepath):
     """
     Write the RST files of ``bpy.msgbus`` module
@@ -2541,6 +2559,7 @@ def rna2sphinx(basepath):
     write_rst_ops_index(basepath)           # `bpy.ops`.
     write_rst_msgbus(basepath)              # `bpy.msgbus`.
     write_rst_geometry_set(basepath)        # `bpy.types.GeometrySet`.
+    write_rst_inline_shader_nodes(basepath)  # `bpy.types.InlineShaderNodes`.
     pyrna2sphinx(basepath)                  # `bpy.types.*` & `bpy.ops.*`.
     write_rst_data(basepath)                # `bpy.data`.
     write_rst_importable_modules(basepath)