docs(readme): link Middleware guide at rendered docs URL

Author: lesnik512

README.md

@@ -88,7 +88,7 @@ async def main() -> None:
         user = await client.get("/users/1", response_model=User)
 ```
 
-Need a custom middleware (auth, tracing, request-ID propagation, etc.)? See the [Middleware guide](docs/middleware.md).
+Need a custom middleware (auth, tracing, request-ID propagation, etc.)? See the [Middleware guide](https://httpware.modern-python.org/middleware/).
 
 ### Streaming responses