Complete developer documentation for MQTT

[mschlenstedt]

Complete developer documentation: https://wiki.loxberry.de/entwickler/mqtt/start

Some documentation can also be found under the wizard documentation (which is the wrong place, but reason is it comes from documentation transfer from the MQTT plugin): https://wiki.loxberry.de/konfiguration/widget_help/widget_mqtt/mqtt_gateway/mqtt_gateway_for_plugin_developers