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

See merge request !5713