Add help section for MM commands

What should this MR do

Check @pedroms' design for the Mattermost service screen: https://gitlab.com/gitlab-org/gitlab-ce/issues/22540

Now this has to be implemented, I've created a clean canvas for you, @lbennett, to draw on.

Does this MR meet the acceptance criteria?

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