Doc: add design proposal process documentation.

Add new document section for processes and new document describing the
design proposal process.

Added CMake support for status handling of design documents.
Improved update process (dependency handling) of Sphinx documentation.
Fixed issue where the intermediate document files from the build
directory were added to Sphinx output if the build directory was placed
under TF-M root directory.

Change-Id: Ie21abe120ffcb5f0a79ddf1c398eb198959878f3
Signed-off-by: Gyorgy Szing <Gyorgy.Szing@arm.com>
5 files changed