Documentation: Cross link to CI/CD docs for Migrating from Jenkins

doc/integration/jenkins.md

 # Jenkins CI service **(STARTER)**
 
+NOTE: **Note:**
+This documentation focuses only on how to **configure** a Jenkins *integration* with
+GitLab. Learn how to **migrate** from Jenkins to GitLab CI/CD in our
+[Migrating from Jenkins](../ci/jenkins/index.md) documentation.
+
 From GitLab, you can trigger a Jenkins build when you push code to a repository, or when a merge
 request is created. In return, Jenkins shows the pipeline status on merge requests widgets and
 on the GitLab project's home page.