doc: Add basic Sphinx configuration and Makefile

Adding the same type of Sphinx config that is used for the main
TF-A repo. Contains the config itself, a Makefile to build the
docs directly under the /docs directory (not tied to top-level
Makefile by design), an index page and the TF.org logo as used
in TF-A.

Change-Id: Ifd5c83533fbad457b448a52f42a2f12303887f4a
Signed-off-by: Paul Beesley <paul.beesley@arm.com>