[docs] Change in release management liaison

Hi all, A belated congrats to Stephen on taking on the docs PTL role! With ramping down my involvement due to changes in $dayjob, we are now also transferring the docs liaison role for release management to Stephen, who kindly volunteered. :) This mostly involves handling docs package release tasks as documented in https://docs.openstack.org/doc-contrib-guide/release/release-tools.html, most notably openstackdocstheme. I've updated https://wiki.openstack.org/wiki/CrossProjectLiaisons accordingly. Thank you, pk