Extending `Building a middleware from scratch` guide

[samueljoli]

The Building a middleware from scratch is a great guide, I think it would be an improvement if the following steps were added:

• Creating a layer

  
  impl TimeoutLayer {
  /// Create a timeout from a duration
  pub fn new(timeout: Duration) -> Self {
      TimeoutLayer { timeout }
  }
  }

impl Layer for TimeoutLayer { type Service = Timeout;

fn layer(&self, service: S) -> Self::Service {
    Timeout::new(service, self.timeout)
}

}

- [Error handling for middleware](https://docs.rs/axum/latest/axum/middleware/index.html#error-handling-for-middleware)

- Using the layer (integration with `ServiceBuilder`