Move plugin_event documentation in docs/plugin_event.

docs/api/conf.py → docs/plugin_event/api/conf.py

@@ -24,7 +24,7 @@ def get_version():
         str: the version number
 
     """
-    with open('../../VERSION') as fi:
+    with open('../../../VERSION') as fi:
         return fi.read()
 
 # If extensions (or modules to document with autodoc) are in another directory,