This moves the completed Stein specs to the implemented directory and
adds the redirects. This was done using:
$ tox -e move-implemented-specs stein
This also removes the stein-template symlink from the docs.
And renames the handling-down-cell_new.rst spec to handling-down-cell.rst
to match the blueprint.
Change-Id: Id92bec8c5a2436a4053765f735d252c7c165f019