docs: stop presenting the in-memory client as the way to connect (#3443)

docs_src/extensions/tutorial003_client.py

@@ -0,0 +1,16 @@
+import anyio
+
+from mcp import Client
+
+
+async def main() -> None:
+    async with Client("http://localhost:8000/mcp") as client:
+        print(client.server_capabilities.extensions)
+        # {'com.example/stamps': {'sealed': True}}
+        result = await client.call_tool("stamp", {"text": "hello"})
+        print(result.content)
+        # [TextContent(type='text', text='[stamped] hello', annotations=None, meta=None)]
+
+
+if __name__ == "__main__":
+    anyio.run(main)