documentation: added create extras folder

Added some code which checks whether the extras folder exists and
creates it if not. This is in response to RTD moaning that it couldn't
find the extras folder all of a sudden.

documentation/conf.py

@@ -420,6 +420,9 @@
 # IMPORTANT: Update this URL when the dei extension is merged in
 base_url = "https://raw.githubusercontent.com/ThreeSixtyGiving/extensions-registry/add-dei-extension/extensions/"
 
+if not os.path.exists("./extras"):
+    os.mkdir("./extras")
+
 if not os.path.exists("./extras/extensions/"):
     os.mkdir("./extras/extensions/")