What does this MR do?
- Update screenshots
- Adds the new feature "issues multiple assignees" to docs
- Improve "18. create new MR" section on issues functionalities
- Remove unnecessary screenshot
closes https://gitlab.com/gitlab-org/gitlab-ee/issues/2440 closes https://gitlab.com/gitlab-org/gitlab-ce/issues/32574
cc/ @axil @SeanPackham