Fix docs warnings

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docs/source/howto/how_to_configure_stock_messaging.rst

 How to configure stock messaging
 ================================
 
-Stock messaging is controlled by an :ref:`availability policy <availabiliy_policies>`
+Stock messaging is controlled by an
+:ref:`availability policy <availability_policies>`
 which is loaded by the :ref:`strategy class <strategy_class>`.
 
 To set custom availability messaging, use your own strategy class to return the

docs/source/topics/key_questions.rst

 * The payment method used.
 
 Recipes:
-* :doc:`/howto/how_to_configure_tax`
+* :doc:`/howto/how_to_apply_tax_exemptions`
 
 What availability messages are shown to customers?
 --------------------------------------------------