Add some more "handler" docs

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

docs/base/handlers/index.rst

@@ -0,0 +1,24 @@
+
+Handlers
+========
+
+Rattail uses the concept of a "handler" to allow for two things primarily:
+
+- customization of app logic without "forking" Rattail package(s)
+- keep logic in a "single" place which can be leverage by multiple UIs
+
+The idea is that for various aspects of an app, there will be one and
+only one "handler" which is responsible for the business logic.
+Rattail usually comes with a default handler, which will be used
+unless you've created your own handler and configured it to be used
+instead.
+
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contents:
+
+   app
+   batch
+   import
+   other/index