Merge branch 'bugfix/docs_rtd_failure_on_missing_submodule_v3_2_bis' into 'release/v3.2'

docs: Add mqtt submodule to RTD build configuration to correct file and delete...

See merge request idf/esp-idf!5310

Author: projectgus

.readthedocs.yml

@@ -14,4 +14,9 @@ formats:
 python:
   version: 2.7
   install:
-    - requirements: docs/requirements.txt
+    - requirements: docs/requirements.txt
+
+# We need to list all the submodules included in documenation build by Doxygen
+submodules: 
+  include:
+    - components/mqtt/esp-mqtt