Fix markdownlint failures under ./roles/ (#7089)

.gitlab-ci/lint.yml

contrib/terraform/gcp/README.md

roles/bootstrap-os/README.md

...es-apps/external_provisioner/cephfs_provisioner/README.md

...s/external_provisioner/local_volume_provisioner/README.md

...netes-apps/external_provisioner/rbd_provisioner/README.md

...-apps/ingress_controller/alb_ingress_controller/README.md

...s/kubernetes-apps/ingress_controller/ambassador/README.md

...kubernetes-apps/ingress_controller/cert_manager/README.md

...ubernetes-apps/ingress_controller/ingress_nginx/README.md

roles/kubernetes-apps/metallb/README.md

roles/kubernetes-apps/registry/README.md