Add initial stock documentation for end users

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

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+# Overview
+
+From the **Stock** page of the Solidus admin interface, you can view and manage
+the stock for all of your store's products.
+
+You can change the [**Stock On Hand**](#stock-on-hand) value for any
+[product][products] (or variant) in your store after selecting the **Edit**
+button next to it. 
+
+Note that you can also [view stock in a product context][product-stock]. 
+
+<!-- TODO: Add screenshot of the stock UI, emphasizing the edit buttons -->
+
+[product-stock]: ../products/product-stock.html
+
+## Stock information
+
+On the **Stock** page, you can see the following information about your stock
+items:
+
+- **Item**: Identifying information about the current item. This information
+    includes the primary product image, the name, and the SKU of the item.
+- **Options**: If the current item is [a variant][variants] of a product, this
+    details which variant it is. For example, which size and color the current
+    item is.
+- **Stock Location**: The [stock location](#stock-locations) of the current
+  item.
+- **Back orderable**: Sets whether the current item should be
+    [backorderable](#backorderable) or not.
+- **Count On Hand**: The number of items that a stock location has of the
+    current item. See [Count on hand](#count-on-hand) for more information.
+
+## Filter and search
+
+You can search for specific stock items. This makes locating stock information
+easier when you have hundreds or thousands of products. You can filter by the
+following values:
+
+- **Stock Location**: Choose one of your existing [stock
+  locations](#stock-locations) to filter results by.
+- **Variant**: You can enter a product's SKU or [option value][option-values] to
+    filter down your results. For example, you could enter a partial SKU to find
+    a range of products that you know have similar SKUs. Or, you could enter
+    "Yellow" to find all of the variants with a "Yellow" option value.
+
+Note that the **Variant** filter searches for option **Name** values
+as well as their **Presentation** values. For example, you may an option value
+with the name "Extra Large" that is presented as "XL" on your storefront. This
+means that you can search for either term and find the same results.
+
+## Stock locations
+
+Stock locations represent a location where your inventory is shipped from. For
+every stock location that you create, a complete set of stock items that
+represents your product collection is also created.
+
+For example, if your store only sells one product, but it ships the product from
+two stock locations, then you should have two items in the list of stock items
+on the **Stock** page. 
+
+<!-- TODO:
+  Add screenshot of two items that represent a single product on the Stock page.
+-->
+
+## Count on hand
+
+For each product you sell in your store, you can manage the **Count On Hand**
+value. You may have inventory for this product at multiple stock locations, and
+you can manage the count for each location.
+
+### Changing the count on hand value
+
+When you update the **Count On Hand** value for a stock item, be aware that you
+are resetting the count.
+
+For example, if your "T-Shirt" product has `5` in stock items and you received
+`15` new shirts from your supplier, you should update the **Count On Hand**
+value to `20`.