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

@@ -0,0 +1,22 @@
+# .readthedocs.yml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Optionally build your docs in additional formats such as PDF and ePub
+formats:
+  - htmlzip
+  - pdf
+
+# Optionally set the version of Python and requirements required to build your docs
+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