Reorder jenkins docs navigation and add new page

Add navigation item to global navigation

New page on docs site: https://docs.gitlab.com/ee/ci/migration/examples/jenkins-maven.html

• Link to the merge request that introduced the new page: gitlab!131446 (merged)

• Give the MR a descriptive title.

• Optional. Link to the review app showing the new page: https://add-jenkins-page-to-nav.docs.gitlab-review.app/ee/ci/migration/plan_a_migration.html

• Related to gitlab#210291 (closed)