doc: Add minimal Sphinx support

Add the essentials for supporting a Sphinx documentation build:

- A makefile under docs/ to invoke Sphinx with the desired output
  format
- A Sphinx master configuration file (conf.py)
- A single, top-level index page (index.rst)
- The TF.org logo that is integrated in the the sidebar of the
  rendered output

Change-Id: I85e67e939658638337ca7972936a354878083a25
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/index.rst b/docs/index.rst
new file mode 100644
index 0000000..a6c1ca9
--- /dev/null
+++ b/docs/index.rst
@@ -0,0 +1,47 @@
+Arm Trusted Firmware-A Documentation Index
+==========================================
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Design Documents
+
+   arm-sip-service
+   auth-framework
+
+   .. toctree::
+   :maxdepth: 1
+   :caption: Release Notes
+   change-log
+
+   .. toctree::
+   :maxdepth: 1
+   :caption: Guides
+
+   .. toctree::
+   :maxdepth: 1
+   :caption: Policies
+   cpu-specific-build-macros
+   exception-handling
+   firmware-design
+   firmware-update
+   interrupt-framework-design
+   platform-compatibility-policy
+   platform-interrupt-controller-API
+   porting-guide
+   psci-lib-integration-guide
+   psci-pd-tree
+   ras
+   reset-design
+   rt-svc-writers-guide
+   sdei
+   secure-partition-manager-design
+   trusted-board-boot
+   user-guide
+   xlat-tables-lib-v2-design
+
+Indices and tables
+==================
+
+* :ref:`genindex`
+* :ref:`modindex`
+* :ref:`search`