Add a large chunk of the docs for command line interface

will have to finish subcommands later

docs/narr/cli/overview.rst

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+
+Overview
+========
+
+The command line interface is an important part of app automation and
+may be thought of in a couple ways:
+
+First there is the :term:`ad hoc script` which is a single file and
+can be placed anywhere, but is not installed as part of a package.
+See :doc:`scripts`.
+
+But the "real" command line interface uses :term:`commands<command>`
+and :term:`subcommands<subcommand>`; these are installed as part of a
+package.
+
+Top-level commands are mostly just a way to group subcommands.  Most
+custom apps would define their own top-level command as well as
+multiple subcommands.  See :doc:`commands` for top-level details.
+
+Subcommands on the other hand are the real workhorse since they define
+the action logic.  See :doc:`subcommands` for more about those.