examples: writing an API for the Apache Kafka: add a root handler for routes documentation to make navigation easier and add some other methods that may find them useful for request state and routes description

Former-commit-id: 3775aab2386051b23e127ccc9e3a6accdfdee6d0

_examples/http_responsewriter/sse/main.go

@@ -114,7 +114,7 @@ func (b *Broker) ServeHTTP(ctx context.Context) {
 		ctx.Writef("data: %s\n\n", <-messageChan)
 		// or json: data:{obj}.
 
-		// Flush the data immediatly instead of buffering it for later.
+		// Flush the data immediately instead of buffering it for later.
 		flusher.Flush()
 	}
 }