Add more docs for Feature Layer

not complete, but progress..

docs/features/custorders/index.rst

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+
+Customer Orders
+===============
+
+Presumably all retailers sell products to customers.  Most will have a
+dedicated point of sale (POS) system for that.
+
+Additionally, some retailers let customers place orders for product
+which will arrive in store for pickup at a later date.  These are
+often referred to as "special orders" or "case orders" etc.
+
+Rattail offers tools to facilitate entry and tracking for such orders,
+providing visibility and workflows for their full life cycle.
+
+.. note::
+   Now that we all know what "COVID" means, online orders with
+   curbside and delivery options have risen in prominence.  For the
+   moment Rattail does not directly provide features with those in
+   mind.  However this "customer orders" feature is clearly related,
+   and probably a lot of the schema and/or logic could be shared
+   between scenarios.  But so far only the classic "special/case
+   order" scenario is being addressed.
+
+.. toctree::
+   :maxdepth: 1
+
+   entry/index
+   purchasing/index
+   receiving/index
+   fulfilment/index
+   cancellation/index
+   reporting/index