Fix default env undecoded topic and doc changes (#1227)

* Fix default env undecoded topic

as BTtoMQTT is not automatically added

* Fix links and doc

platformio.ini

@@ -423,7 +423,7 @@ build_flags =
   '-DLED_SEND_RECEIVE=2'
   '-DLED_SEND_RECEIVE_ON=0'
   '-DGateway_Name="OpenMQTTGateway_ESP32_BLE"'
-  '-DMQTTDecodeTopic="BTtoMQTT/undecoded"'
+  '-DMQTTDecodeTopic="undecoded"'
 
 [env:esp32dev-ble-aws]
 platform = ${com.esp32_platform}