docs: Update Hierarchial Multi Agent Example (#3282)

Was trying to learn the Multi Agent Workflow examples and encountered
some errors, which I fixed by editing these:

* Added missing state for Team1, and importing `Command`
* `ValueError: Node `LangGraph` already present.`: Seems to happen we
add the `team_1_graph` node without giving it a name, it will default to
the name `LangGraph`. Solved by giving the sub-graph a name when
building the top-level supervisor.
* Added the edges for the graph to feedback to the top level supervisor
to decide whether it still needs to relegate the task to other nodes or
end from there

---------

Co-authored-by: Vadym Barda <vadim.barda@gmail.com>

docs/docs/concepts/multi_agent.md

@@ -241,7 +241,7 @@ To address this, you can design your system _hierarchically_. For example, you c
 from typing import Literal
 from langchain_openai import ChatOpenAI
 from langgraph.graph import StateGraph, MessagesState, START, END
-
+from langgraph.types import Command
 model = ChatOpenAI()
 
 # define team 1 (same as the single supervisor example above)
@@ -286,7 +286,7 @@ team_2_graph = team_2_builder.compile()
 # define top-level supervisor
 
 builder = StateGraph(MessagesState)
-def top_level_supervisor(state: MessagesState):
+def top_level_supervisor(state: MessagesState) -> Command[Literal["team_1_graph", "team_2_graph", END]]:
     # you can pass relevant parts of the state to the LLM (e.g., state["messages"])
     # to determine which team to call next. a common pattern is to call the model
     # with a structured output (e.g. force it to return an output with a "next_team" field)
@@ -297,10 +297,11 @@ def top_level_supervisor(state: MessagesState):
 
 builder = StateGraph(MessagesState)
 builder.add_node(top_level_supervisor)
-builder.add_node(team_1_graph)
-builder.add_node(team_2_graph)
-
+builder.add_node("team_1_graph", team_1_graph)
+builder.add_node("team_2_graph", team_2_graph)
 builder.add_edge(START, "top_level_supervisor")
+builder.add_edge("team_1_graph", "top_level_supervisor")
+builder.add_edge("team_2_graph", "top_level_supervisor")
 graph = builder.compile()
 ```