Move Service Desk doc to own folder

doc/administration/encrypted_configuration.md

doc/administration/incoming_email.md

doc/administration/raketasks/service_desk_email.md

doc/administration/silent_mode/index.md

doc/api/users.md

doc/development/features_inside_dot_gitlab.md

doc/development/internal_users.md

doc/subscriptions/bronze_starter.md

doc/subscriptions/gitlab_com/index.md

doc/subscriptions/self_managed/index.md

doc/user/crm/index.md

doc/user/gitlab_com/index.md

doc/user/project/description_templates.md

doc/user/project/issues/create_issues.md

doc/user/project/service_desk.md

...ject/service_desk/img/service_desk_confirmation_email.png

...r/project/service_desk/img/service_desk_issue_tracker.png

doc/user/project/service_desk/img/service_desk_reply.png

doc/user/project/service_desk/img/service_desk_thread.png

doc/user/project/service_desk/index.md