Merge pull request #3105 from doctrine/feature/docs-cleanup

Add sidebar.rst and cleanup docs homepage.

docs/en/index.rst

 DBAL Documentation
 ==================
 
-Welcome to the Doctrine Database Abstraction Layer documentation.
+The Doctrine DBAL documentation is a reference guide to everything you need
+to know about the database abstraction layer.
 
-.. toctree::
-   :depth: 2
-   :glob:
+Getting Help
+------------
 
-   *
+If this documentation is not helping to answer questions you have about the
+Doctrine DBAL, don't panic. You can get help from different sources:
+
+-  Gitter chat room `#doctrine/dbal <https://gitter.im/doctrine/dbal>`_
+-  On `Stack Overflow <http://stackoverflow.com/questions/tagged/doctrine-dbal>`_
+-  The `Doctrine Mailing List <http://groups.google.com/group/doctrine-user>`_
+-  Report a bug on `GitHub <https://github.com/doctrine/dbal/issues>`_.
+
+Getting Started
+---------------
+
+The best way to get started is with the :doc:`Introduction <reference/introduction>` section
+in the documentation. Use the sidebar to browse other documentation for the Doctrine PHP DBAL.

docs/en/sidebar.rst

+.. toctree::
+    :depth: 3
+
+    reference/introduction
+    reference/architecture
+    reference/configuration
+    reference/data-retrieval-and-manipulation
+    reference/query-builder
+    reference/transactions
+    reference/platforms
+    reference/types
+    reference/schema-manager
+    reference/schema-representation
+    reference/events
+    reference/security
+    reference/sharding
+    reference/sharding_azure_tutorial
+    reference/supporting-other-databases
+    reference/portability
+    reference/caching
+    reference/known-vendor-issues