docs: Add mqtt submodule to RTD build configuration. Now any submodule included in documentation build by Doxygen should be included in this file

.readthedocs.yml

@@ -15,3 +15,8 @@ python:
   version: 2.7
   install:
     - requirements: docs/requirements.txt
+
+# We need to list all the submodules included in documenation build by Doxygen
+submodules: 
+  include:
+    - components/mqtt/esp-mqtt