Follow-up from "Add markdownlint-cli to yarn dependencies"

There are few points that came out of the discussion in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32593#note_211879571:

• Do we really need to pin down the markdownlint version? It's used in two places now, in the Docker image that is used to run the docs lint job, and in packages.json.
• Should we advocate for users to use a Docker command to run the linter instead of using yarn?