Add docs for handlers, app handler

docs/narr/handlers/overview.rst

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+
+Overview
+========
+
+The :term:`handler` concept is central to the :term:`app` architecture.
+
+They are analogous to "plugins" but only *one* handler is used for a
+given purpose.  See :doc:`arch`.
+
+So far there is only one handler type defined; see :doc:`app`.