[DOCS] Updated master_doc in conf.py

Due to a mismatch in Sphinx versions being used by the development 
machine and Read the Docs, this is necessary to specify that the the 
index.rst file contains the root toctree directive.

docs/conf.py

@@ -24,6 +24,8 @@
 
 # -- General configuration ---------------------------------------------------
 
+master_doc = 'index'
+
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
@@ -49,4 +51,4 @@
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-html_static_path = ['_static']
+html_static_path = ['_static']