Add more docs for Feature Layer

not complete, but progress..

docs/features/purchasing/ordering/overview.rst

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+
+Overview
+========
+
+Ordering is when the retailer expresses to the vendor, their interest
+in purchasing some of their product for resale to their customers.
+The retailer promises to pay for the product, so the vendor should
+ship it to them ASAP.
+
+If the retailer has things "dialed in" then ordering may be fully or
+partially automatic, aka. "suggested ordering" where the purchase
+order is auto-generated based on current inventory levels.
+
+Rattail does not (yet?) offer any tools for suggested ordering.  It
+does though offer tools for the more classic scenario, where the buyer
+"manually" assembles the purchase order.
+
+It does this using a batch.  The buyer creates a new Ordering batch
+for the given vendor, and then adds items to it.
+
+Adding items may be done in a few ways.  These are discussed in
+:doc:`additems`.
+
+One way or another the batch is populated, then ultimately executed.
+Read more about that in :doc:`execution`.
+
+We also share some thoughts here, on :doc:`convertpo`.