Add markdown CI (#5380)

.gitlab-ci/lint.yml

.markdownlint.yaml

README.md

docs/ansible.md

docs/arch.md

docs/atomic.md

docs/aws.md

docs/azure.md

docs/calico.md

docs/cinder-csi.md

docs/cloud.md

docs/cni.md

docs/comparisons.md

docs/contiv.md

docs/coreos.md

docs/cri-o.md

docs/debian.md

docs/dns-stack.md

docs/downloads.md

docs/flannel.md