WIP: Introdue markdownlint to check documentation markdown styleguide

What does this MR do?

Provide an automated way to check the documentation markdown styleguide.

Why was this MR needed?

To reduce workload for documentation reviewers.

What are the relevant issue numbers?

Partially closes https://gitlab.com/gitlab-org/gitlab-ce/issues/3909

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together