Add docs for class-based handlers

aiogram/__init__.py

@@ -1,7 +1,7 @@
 from .api import methods, types
 from .api.client import session
 from .api.client.bot import Bot
-from .dispatcher import filters
+from .dispatcher import filters, handler
 from .dispatcher.dispatcher import Dispatcher
 from .dispatcher.router import Router
 
@@ -23,6 +23,7 @@ __all__ = (
     "Dispatcher",
     "Router",
     "filters",
+    "handler",
 )
 
 __version__ = "3.0.0a0"