add rhel8-cern.repo file

docs/rhel/repofiles/rhel8-cern.repo

+[cern]
+name=RHEL $releasever - CERN
+baseurl=http://linuxsoft.cern.ch/cern/centos/s8/CERN/$basearch
+enabled=1
+priority=5
+exclude=centos-linux-release,centos-stream-release
+gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-kojiv2
+gpgcheck=1