modify file structure, add mqtt version readme

Author: linomp

arduino_secrets _TEMPLATE.h coap_version/arduino_secrets _TEMPLATE.h

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+# MQTT Version
+
+This version of the arduino code communicates through MQTT.
+
+## Required Libraries:
+- [WiFiNINA](https://www.arduino.cc/reference/en/libraries/wifinina/)
+- [ArduinoMqttClient](https://www.arduino.cc/reference/en/libraries/arduinomqttclient/)
+- [STM32duino X-NUCLEO-IKS01A3](https://www.arduino.cc/reference/en/libraries/stm32duino-x-nucleo-iks01a3/3)
+
+
+## How to test:
+
+1. Download the [Mosquitto CLI](https://mosquitto.org/download/) (available for Windows, Linux, etc.).
+
+2. Launch an mqtt client and connect to the predefined broker & topic like this:
+
+    ```
+    mosquitto_sub -h "apps.xmp.systems" -t "iiot-project/test"
+    ```

rp2040_webthing.ino coap_version/coap_version.ino

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+#define SECRET_SSID ""
+#define SECRET_PASS ""