Adds r/stx.5.0 to the list of branches to promote for starlingx docs.
Adds r/stx.5.0 to the list of branches to promote for starlingx docs.

Signed-off-by: Kevin Putnam <8adc7edcac770c9413d58ffbae867d0245fc20d5@intel.com>
Change-Id: Ic1722c158d0f904cc7eb10bd09670725f3037dad

openstack-infra/project-config,openstack-infra/project-config