[ADD] ecommerce: How to show product availability

ecommerce/managing_products/stock.rst

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+---------------------------------
+How to show product availability
+---------------------------------
+
+The availability of your products can be shown on the website to reassure your customers.
+
+.. image:: ./media/stock_web.png
+   :align: center
+
+To display this, open the *Sales* tab in the product detail form and select an option in 
+*Availability*.
+
+.. image:: ./media/stock.png
+   :align: center
+
+A custom warning message can be anything related to a stock out, delivery delay, etc.
+
+.. image:: ./media/stock_warning.png
+   :align: center
+
+.. note::
+    This tool does not require the Inventory app to be installed.
+
+.. tip::
+    If one item is no longer sellable, unpublish it from your website. If it comes to
+    one particular product variant, deactivate the variant in the backend (see :doc:`pricelists`).