docs: add Sphinx based documentation

Add Sphinx based documentation and the infrastructure to build documentation.
Along with this add overview and place holders for user guide.

Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Tianyu Li <Tianyu.Li@arm.com>
Reviewed-by: Nathan Brown <Nathan.Brown@arm.com>

Change-Id: I14c705af4c148b3fbbc0b779af2c1e1e0f108d7f