ESP-MQTT is a standard ESP-IDF component. Please refer to instructions in ESP-IDF
Please refer to the standard ESP-IDF, documentation for the latest version: https://docs.espressif.com/projects/esp-idf/
Documentation of ESP-MQTT API: https://docs.espressif.com/projects/esp-idf/en/latest/api-reference/protocols/mqtt.html
For ESP-IDF versions prior to IDFv3.2, please clone as a component of ESP-IDF:
git submodule add https://github.com/espressif/esp-mqtt.git components/espmqtt
and checkout the ESP-MQTT_FOR_IDF_3.1 tag