[training-guides] improve the index.rst file

Default auto-generated index file is pretty much useless.
This patch makes index similar to Upstream's.

Change-Id: If17faaa47833b2b5044c4d885af25d97d17e9cf4

doc/training-guides/associate-guide/index.rst

@@ -1,18 +1,21 @@
+===========================================
 OpenStack Training Guides - Associate Guide
 ===========================================
 
-Contents:
+.. image:: ../figures/os_background.png
+   :class: fill
+   :width: 100%
 
-.. toctree::
-   :maxdepth: 1
+Modules
+=======
 
-   02-getting-started-quiz
-   06-compute-node-quiz
-   07-associate-network-node
+- `OpenStack networking <07-associate-network-node.html>`_
+
+
+Quizzes
+=======
+
+- `Getting started <02-getting-started-quiz.html>`_
+- `Compute node <06-compute-node-quiz.html>`_
 
-Indices and tables
-==================
 
-* :ref:`genindex`
-* :ref:`modindex`
-* :ref:`search`