docs: remove non-existent astream() method and fix concurrent agents example (#103)

- Remove 'Async Streaming' section that documented astream() method which was never implemented
- Update 'Concurrent Agents' example to use actual functions (run_conversation, callback) from the example code instead of undefined run_task()
- Properly demonstrate concurrent execution using loop.run_in_executor() with asyncio.gather()

Co-authored-by: openhands <openhands@all-hands.dev>

Author: neubig

sdk/guides/convo-async.mdx

@@ -116,30 +116,20 @@ cd agent-sdk
 uv run python examples/01_standalone_sdk/11_async.py
 ```
 
-### Async Streaming
-
-Use `astream()` to process events as they occur without blocking:
-
-```python highlight={4-5}
-async def run_agent():
-    conversation = Conversation(agent=agent, workspace=cwd)
-    conversation.send_message("Write 3 facts about Python to FACTS.txt")
-    
-    async for event in conversation.astream():
-        print(f"Event: {event}")
-```
-
 ### Concurrent Agents
 
 Run multiple agent tasks in parallel using `asyncio.gather()`:
 
-```python highlight={4-7}
+```python highlight={10-14}
 async def main():
-    # Create multiple conversation tasks
+    loop = asyncio.get_running_loop()
+    callback = AsyncCallbackWrapper(callback_coro, loop)
+
+    # Create multiple conversation tasks running in parallel
     tasks = [
-        run_task("task 1"),
-        run_task("task 2"),
-        run_task("task 3")
+        loop.run_in_executor(None, run_conversation, callback),
+        loop.run_in_executor(None, run_conversation, callback),
+        loop.run_in_executor(None, run_conversation, callback)
     ]
     results = await asyncio.gather(*tasks)
 ```