Fix tox docs failure
Fix tox docs failure

doc/source/_static directory is required by oslo.policy sample generator.
_static directory is empty in the cinder repo but it must exist.
https://review.opendev.org/#/c/571106/ dropped doc/source/_static/.placeholder
accidentally. This commit recovers the place holder file to ensure
doc/source/_static directory exists.

Closes-bug: #1841131
Co-Authored-By: Akihiro Motoki <e9c22966286dfb7b63536b5480a1a395590f8876@gmail.com>
Change-Id: Ifad4371afa096afc2d0d71f6f077a53a9e52b60a

mahak/cinder,openstack/cinder,mahak/cinder,openstack/cinder