Documentation for Issue Boards

What does this MR do?

Add documentation for issue boards.

What are the relevant issue numbers?

• Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/17907
• Backend MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5548
• Frontend MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5554