Add initial docs for Customer Order system features

docs/data/custorders/index.rst

@@ -0,0 +1,19 @@
+
+Customer Orders
+===============
+
+Rattail has a "customer order" system of sorts.  I'm calling it a
+"customer order" system but most folks will probably think of it as
+e.g. a "special order" system or "case order" system etc.
+
+This section describes the "customer order" features of Rattail.
+
+See the online `Theo demo`_ to play around with this concept.  It has
+access to a CORE-POS system, from which it gets customer/product data.
+
+.. _Theo demo: https://demo-theo.rattailproject.org/
+
+.. toctree::
+
+   overview
+   workflows