Prefix MQTT demo doc sections with "MQTT" (#1261)
Prefix MQTT demo doc sections with "MQTT" (#1261)

* Prefix MQTT demo doc sections with "MQTT" in preparation for HTTP demos of the same name.

* Add oxford comma and fix name of shadow library in description.
aws/aws-iot-device-sdk-embedded-C,aws/aws-iot-device-sdk-embedded-C,aws/aws-iot-device-sdk-embedded-C