[requirements][FFE][openstackdocstheme] Feature freeze exception for openstackdocstheme

Hello. I would like to request a freeze exception to release openstackdocstheme per [1]. This is more on the basis of "extremely low risk" than "gotta have it": it picks up a couple of commits with aesthetic improvements I would like to take advantage of (see commit message). Thanks, efried [1] https://review.openstack.org/642908