Revert "the mistral team deleted their admin guide landing page"
Revert "the mistral team deleted their admin guide landing page"

This reverts commit c6831022ff00aad3c8ff3b41ef75f4046b47b88c.

Change-Id: I1fa92ffbaaa3839b87b7e42c3f756f2878f1e7f4
Depends-on: I821257f5be8b6778b1a6eaf9c138a06cef8b2792
openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals