Merge branch 'bugfix/docs_rtd_failure_on_missing_submodule_v3_3' into 'release/v3.3'

docs: Add mqtt submodule to RTD build configuration. Now any submodule...

See merge request idf/esp-idf!5285

.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