update doc-build.sh to allow multiple depth of dir

update doc-build.sh to allow multiple depth of dir

In our new documentation guideline, we suggest to create
documentation directory just in one depth from top 'docs' directory.
This rule can make sure there is no name collision.

However, some projects are already using multiple depth of
documentation directories. So, let's allow them to create their
documents without any namespace collision by creating unique names
from directory path.

Change-Id: I7806abd00be56b0a4811b3d17216495e11bca825
Signed-off-by: Ryota MIBU <r-mibu@cq.jp.nec.com>