Add markdown CI (#5380)

docs/getting-started.md

docs/ha-mode.md

docs/integration.md

docs/kube-ovn.md

docs/kube-router.md

docs/kubernetes-reliability.md

docs/large-deployments.md

docs/macvlan.md

docs/multus.md

docs/netcheck.md

docs/opensuse.md

docs/packet.md

docs/proxy.md

docs/recover-control-plane.md

docs/roadmap.md

docs/test_cases.md

docs/upgrades.md

docs/vagrant.md

docs/vars.md

docs/vsphere.md