[DOCS] Add recommendation for EMQX (#1641)

* [DOCS] Add recommendation for EMQX

* [DOCS] Adjust the position and size of the sponsor logo

docs/prerequisites/broker.md

@@ -6,6 +6,7 @@ All the events or commands could pass by the broker.
 There are many choices of brokers, here are some of the most popular:
 * [Mosquitto](https://mosquitto.org/) (Open source)
 * [Moquette](https://moquette-io.github.io/moquette/) (Open source)
+* [EMQX](https://www.emqx.io/) (Open Source)
 * [HiveMQ](https://www.hivemq.com/hivemq/features/)
 * Embedded MQTT brokers (Home Assistant and OpenHAB)