Add Jenkins overview and use cases
Add Jenkins overview and use cases to https://docs.gitlab.com/ee/integration/jenkins.html
Follow up from https://gitlab.com/gitlab-org/gitlab-ee/issues/2703#note_35588317
Add Jenkins overview and use cases to https://docs.gitlab.com/ee/integration/jenkins.html
Follow up from https://gitlab.com/gitlab-org/gitlab-ee/issues/2703#note_35588317