fix(mypy): Use slice assignment for docutils node children manipulation

Replace .clear() + .extend() with slice assignment (node[:] = children)
to fix mypy errors about Sequence[Node] having no .clear() method and
Node having no .extend() method. This is the idiomatic pattern used in
Sphinx's codebase for modifying node children.

Author: tony

docs/_ext/argparse_exemplar.py

@@ -823,8 +823,7 @@ def process_node(
             else:
                 new_children.append(child)
         if children_changed:
-            node.children.clear()
-            node.extend(new_children)
+            node[:] = new_children  # type: ignore[index]
 
     return node
 
@@ -1134,8 +1133,7 @@ def _extract_sections_from_container(
             remaining_children.append(child)
 
     # Update container with remaining children only
-    container.children.clear()
-    container.extend(remaining_children)
+    container[:] = remaining_children  # type: ignore[index]
 
     return container, extracted_sections