docs: Add mopidy.ext to API docs

docs/api/ext.rst

@@ -0,0 +1,9 @@
+.. _ext-api:
+
+*************
+Extension API
+*************
+
+.. automodule:: mopidy.ext
+    :synopsis: Extension API for extending Mopidy
+    :members:

docs/api/index.rst

@@ -11,4 +11,5 @@ API reference
     core
     audio
     frontends
+    ext
     http

mopidy/ext.py

@@ -11,28 +11,35 @@ logger = logging.getLogger('mopidy.ext')
 
 
 class Extension(object):
+    """Base class for Mopidy extensions"""
 
     dist_name = None
     ext_name = None
     version = None
 
     def get_default_config(self):
+        """TODO"""
         raise NotImplementedError(
             'Add at least a config section with "enabled = true"')
 
     def get_config_schema(self):
+        """TODO"""
         return config_utils.ExtensionConfigSchema()
 
     def validate_environment(self):
+        """TODO"""
         pass
 
     def get_frontend_classes(self):
+        """TODO"""
         return []
 
     def get_backend_classes(self):
+        """TODO"""
         return []
 
     def register_gstreamer_elements(self):
+        """TODO"""
         pass