Add initial order documentation for end users

This commit adds an overview that explains the most basic elements of a
Solidus order.

Note that there are TODOs in the text that should be dealt with when
more end user guides have been merged.

guides/data/nav/users.yml

@@ -1,4 +1,9 @@
 -
+  - title: "Orders"
+    dropdown:
+      - title: "Overview"
+        href: "/users/orders/overview.html"
+
   - title: "Products"
     dropdown:
       - title: "Overview"

guides/source/users/orders/overview.html.md

@@ -0,0 +1,63 @@
+# Orders
+
+Orders are a key part of Solidus. The orders user interface is especially
+complex because orders connect many of the other major parts
+of the Solidus admin – like payments, shipments, taxes, and customer profiles.
+
+To make sense of orders, this section of the guide explains orders in the
+context of a typical ecommerce store that sells basic physical products (like
+t-shirts), and it links out to other related parts of the users guide.
+
+## What is an order?
+
+Every time that a customer orders product(s) from your store, a new order is
+created. But Solidus starts tracking orders *before* they are placed: as soon as
+a customer has put something in their cart, a new order is generated.
+
+There are many related database objects that "make up" an order, and are created
+in service of an order:
+
+<!-- TODO:
+  Add links to other end-user documentation after it has been merged.
+-->
+
+- **[Customer][users]**: Every order requires an associated customer (also called a
+  *user*.
+- **Shipments**: An order may have one or many shipments associated with it.
+- **Adjustments**: Additional charges (taxes and shipping fees) and credits
+  ([promotional discounts][promotions]) are tied to the order.
+- **Payments**: An order may have one or more payments associated with it. For
+  example, a customer may use a gift card and a credit card to make a purchase.
+  They might also require a refund if a product is defective.
+- **RMAs and returns**: If a customer decides to return an order for some
+  reason, return authorizations, return items, and reimbursements are created
+  and tracked against the original order.
+
+All of these objects can be viewed in the context of the order – or in their own
+sections of the Solidus admin.
+
+[users]: ../users/overview.html
+[promotions]: ../promotions/overview.html
+
+## Order states
+
+Before an order is marked as **Complete**, orders go through a number of other
+states. By default, the following states are available:
+
+- **Cart**: A customer has placed product(s) in the cart.
+- **Address**: The customer has begun checkout and has not yet provided their
+  billing and/or shipping addresses.
+- **Delivery**: The customer needs to choose from available shipping options.
+- **Payment**: The customer needs to provide payment information.
+- **Confirm**: The customer needs to confirm that the input checkout information
+  is correct.
+- **Complete**: The customer has submitted their order.
+
+<!-- TODO:
+  Link to payments and shipments state machine documentation for end users once
+  it has been merged.
+-->
+
+Once an order has been completed, payments and shipments can start to be
+processed by your store. Note that both payments and shipments have their own,
+separate state sets.