doc: Treat Sphinx warnings as errors ...

'make doc' will now fail if Sphinx outputs any warning messages during
documentation generation.

Change-Id: I3e466af58ccf29b14a7e61037539b79ab6fc6037
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

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>