Add markdownlint-cli to yarn dependencies

What does this MR do?

This is the tool that we use to lint the Markdown files. Let's add it to yarn.lock so that we can use it in development. Pinned to 0.18.0.

When this is installed locally, we'll be able to run:

yarn run markdownlint

To get an idea how this works, you can check the new guidelines https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32273/diffs.

PS. I'm working on creating some docs raketasks and this is an MVC for that.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content