add readthedocs yaml configuration file

doc/conf.py

@@ -194,7 +194,7 @@
 # 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 = ['css', 'fig']
+html_static_path = ['css', 'fig', '_static']
 
 # These paths are either relative to html_static_path
 # or fully qualified paths (eg. https://...)