Docs: Fix duplicate object description for AbstractOrder

If using autodoc to document Oscar app's models, one needs to add
:noindex: so Sphinx knows which page has the canonical model
documentation.

docs/source/howto/how_to_set_up_order_processing.rst

@@ -89,9 +89,11 @@ methods to make this easier.
 
 .. autoclass:: oscar.apps.order.abstract_models.AbstractOrder
    :members: pipeline, all_statuses, available_statuses, set_status
+   :noindex:
 
 .. autoclass:: oscar.apps.order.abstract_models.AbstractLine
    :members: pipeline, all_statuses, available_statuses, set_status
+   :noindex:
 
 Example
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