Fix example using add_config_value (#10937)

``add_config_value()`` has mandatory 3 arguments.

doc/development/theming.rst

@@ -328,7 +328,7 @@ code may use:
     # We connect this function to the step after the builder is initialized
     def setup(app):
         # Tell Sphinx about this configuration variable
-        app.add_config_value('my_javascript_variable')
+        app.add_config_value('my_javascript_variable', 0, 'html')
         # Run the function after the builder is initialized
         app.connect('builder-inited', add_js_variable)