Add a section to the docs how to use Zigbee2MQTT

[syssi]

I currently run a docker host which has Home Assistant, ESPHome, Zigbee2MQTT and Mosquitto Broker to name the important bits. Zigbee devices are predominantly attached to a ConBee II that is attached to the server and mounted to Zigbee2MQTT, though I've now added the GW-03 also, with a 2nd Zigbee2MQTT instance, same broker.

The "Add the ESPHome node to Home Assistant" section in the docs is slightly different for me as I don't use the HASS integration and haven't fixed an mDNS issue that sees me chasing static IPs 🥺

The "Connect the device to the ZHA integration of Home Assistant" section in the docs has all the information except that for Zigbee2MQTT you can either go to the application itself and setup the adapter as follows:

I will have configured via the config files however, which is an entry of the following:

serial:
  port: tcp://192.168.1.100:6638
  adapter: ezsp

Official docs can be found here: https://www.zigbee2mqtt.io/guide/configuration/adapter-settings.html

If you like I can take a stab at a PR to list out both ZHA and Zigbee2MQTT options?

Originally posted by @cptbucky in #18 (comment)