Link to local copy of cliff and stevedore docs

We used to host documentation for cliff and stevedore on
readthedocs.org, but have updated the publishing jobs so that the docs
are hosted on docs.openstack.org with the other projects. This change
updates the links to the documentation on the main page listing all of
the projects.

Change-Id: Ibe5f01c6fee57353fcd8cd187e6894247ff88a56