feat: add basic "create order" feature, docs, tests

just the package API docs so far, narrative will come later

docs/glossary.rst

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+
+Glossary
+========
+
+.. glossary::
+   :sorted:
+
+   order
+     This is the central focus of the app; it refers to a customer
+     case/special order which is tracked over time, from placement to
+     fulfillment.  Each order may have one or more :term:`order items
+     <order item>`.
+
+   order item
+     This is effectively a "line item" within an :term:`order`.  It
+     represents a particular product, with quantity and pricing
+     specific to the order.
+
+     Each order item is tracked independently of its parent order and
+     sibling items.
+
+   pending customer
+     Generally refers to a "new / unknown" customer, e.g. for whom a
+     new order is being created.  This allows the order lifecycle to
+     get going before the customer has a proper account in the system.
+
+     See :class:`~sideshow.db.model.customers.PendingCustomer` for the
+     data model.
+
+   pending product
+     Generally refers to a "new / unknown" product, e.g. for which a
+     new order is being created.  This allows the order lifecycle to
+     get going before the product has a true record in the system.
+
+     See :class:`~sideshow.db.model.products.PendingProduct` for the
+     data model.