Add some more "handler" docs

app handler, batch handlers, native batch types, people handler

docs/base/handlers/app.rst

@@ -0,0 +1,23 @@
+
+App Handler
+===========
+
+There is one handler which corresponds to the app itself, whereas all
+other handlers correspond to some "portion" of the app.  This main app
+handler's primary purpose is to:
+
+- provide a common interface for obtaining all other handlers
+- provide some "global" methods, e.g. for rendering date/time values
+
+You can override the main app handler, with config like:
+
+.. code-block:: ini
+
+   [rattail]
+   app.handler = poser.app:PoserAppHandler
+
+Of course that must point to a valid ``AppHandler`` class, which you
+probably must create.
+
+See also :class:`rattail:rattail.app.AppHandler` which is the default
+and base class.