This adds specs/mitaka/* to the doc index and
creates the specs/mitaka itself with the dummy spec.
Change-Id: Iadf4a7d4ed1b7df7185428ce123275d325bf0608
The previously approved GBP spec is being filed here.
(Earlier gerrit review: https://review.openstack.org/#/c/89469)
Change-Id: Ic5e212e58ff4bdfbe211eaa936ad96b0fcfbf79f