[MOV] tutorials/getting_started/*: promote advanced chapters to independent tutorials

task-2991663

Part-of: odoo/documentation#3644

content/developer/tutorials/mixins.rst

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+======================
+Reuse code with mixins
+======================
+
+If you need to interface with common Odoo features such as the chatter, you can rely on
+:doc:`mixins <../reference/backend/mixins>`.
+They are Odoo models exposing useful methods through inheritance.
+
+To learn and play with mixins, visit `this repository <https://github.com/tivisse/odoodays-2018/>`_.
+This module for a plant nursery is training material developed for the OXP 2018. You don't need to
+code it on your side. But you can check the presentations in the :file:`/static/pdf` directory and
+play with the module to discover some magic features in Odoo.