[docs] Request to clean up reviewers on openstack-doc-core and openstack-contributor-guide-core

Greetings, I went to go hunt down a reviewer for the contributors guide this morning and found that at least one of the reviewers no longer works on OpenStack. In this case pkovar, but just glancing at the group lists, there appears to be others. If someone with appropriate privileges could update the group memberships to better reflect reality, it would be helpful to the rest of the community. Thanks! -Julia