Overview and use cases for Service Desk
What does this MR do?
Adds overview and use cases for Service Desk documentation.
For https://gitlab.com/gitlab-org/gitlab-ee/issues/2703
See also https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/6685 (Service Desk webpage)
Edited by 🚄 Job van der Voort 🚀