Fixed Breathe version to avoid docs build failure on ReadTheDocs server. Provided RTD project configuration file to be able to select python version for specific to esp-idf release.

.readthedocs.yml

@@ -0,0 +1,17 @@
+# .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

docs/requirements.txt

@@ -1,10 +1,11 @@
 # This is a list of python packages used to generate documentation. This file is used with pip:
 # pip install --user -r requirements.txt
 #
-sphinx>=1.6.5
+sphinx>=1.8.4
+breathe==4.11.1
 sphinx-rtd-theme
-breathe>=4.7.3
-sphinxcontrib-blockdiag>=1.5.3
+sphinx-notfound-page
+sphinxcontrib-blockdiag>=1.5.5
 sphinxcontrib-seqdiag>=0.8.5
 sphinxcontrib-actdiag>=0.8.5
 sphinxcontrib-nwdiag>=0.9.5