[github-actions] add markdown lint check (#6669)
[github-actions] add markdown lint check (#6669)

openthread/openthread,abtink/openthread,abtink/openthread,srickardti/openthread,srickardti/openthread,srickardti/openthread,srickardti/openthread,openthread/openthread,abtink/openthread,jwhui/openthread,jwhui/openthread,openthread/openthread,jwhui/openthread,abtink/openthread,jwhui/openthread,openthread/openthread