add invocation section to langchain/agents guide (langchain-ai#893)

Author: ccurme

src/oss/langchain/agents.mdx

@@ -529,6 +529,29 @@ const result = await agent.invoke(
 For more details on message types and formatting, see [Messages](/oss/langchain/messages). For comprehensive middleware documentation, see [Middleware](/oss/langchain/middleware).
 </Tip>
 
+## Invocation
+
+You can invoke an agent by passing an update to its [state](/oss/langgraph/graph-api#state). All agents include a [sequence of messages](/oss/langgraph/use-graph-api#messagesstate) in their state; to invoke the agent, pass a new message:
+
+:::python
+```python
+result = agent.invoke(
+    {"messages": [{"role": "user", "content": "What's the weather in San Francisco?"}]}
+)
+```
+:::
+:::js
+```typescript
+await agent.invoke({
+  messages: [{ role: "user", content: "What's the weather in San Francisco?" }],
+})
+```
+:::
+
+For streaming steps and / or tokens from the agent, refer to the [streaming](/oss/langchain/streaming) guide.
+
+Otherwise, the agent follows the LangGraph [Graph API](/oss/langgraph/use-graph-api) and supports all associated methods.
+
 ## Advanced concepts
 
 ### Structured output