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

mentioned in issue #2944

mentioned in merge request !2524 (merged)

marked this issue as related to #2703 (closed)

marked this issue as related to #2944

closed via merge request !2524 (merged)

mentioned in commit 3a1b9c6a