Fix some Sphinx warnings

Author: Ayuto

addons/source-python/docs/source-python/source/_static/readme.md

@@ -0,0 +1 @@
+Custom static files (such as style sheets or script files) will be placed here.

addons/source-python/docs/source-python/source/conf.py

@@ -44,7 +44,7 @@
 # The suffix(es) of source filenames.
 # You can specify multiple suffix as a list of string:
 # source_suffix = ['.rst', '.md']
-source_suffix = '.rst'
+source_suffix = {'.rst': 'restructuredtext'}
 
 # The encoding of source files.
 #source_encoding = 'utf-8-sig'
@@ -71,7 +71,7 @@
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = 'en'
 
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used: