Fix broken URLs
Fix broken URLs

http://files.openstack.org/docs-404s/2020-02-21.txt showed
that we had invalid URLs like
//www.openstack.org/community/events/

fix this one and friends.

Change-Id: I6e0a62efd0aae8b0cd2e76da01cca08290934bf1

openstack/openstackdocstheme,openstack/openstackdocstheme,openstack/openstackdocstheme,openstack/openstackdocstheme