Link to API version history from main index
Link to API version history from main index

It's not only contributors that care about the v3
microversion history, it's also people writing to the
API, so this brings the version history doc to the main
index as a link.

Change-Id: Ifc951fc83ee0c94fe25fc24f36440c70c559b31c

mahak/cinder,phenoxim/cinder,eharney/cinder,mahak/cinder,Datera/cinder,eharney/cinder,j-griffith/cinder,Datera/cinder,j-griffith/cinder,openstack/cinder,openstack/cinder,phenoxim/cinder