docs/overview: update code structure description

build directory was created to store intermediate files during
compilation or solution execution. As it's not used and removed, so
update the document accordingly.

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

Change-Id: Icc280e0ec5a67a7a4fc6315cea921166f865c7fb