Move jenkins doc to migrations folder

Moved the jenkins migration doc into the migration
folder, where it belonds. Other related docs will
be located here too, like CircleCI and GitHub Actions.

doc/ci/README.md

@@ -63,7 +63,7 @@ the following documents:
 If you're migrating from another CI/CD tool, check out our handy references:
 
 - [Migrating from CircleCI](migration/circleci.md)
-- [Migrating from Jenkins](jenkins/index.md)
+- [Migrating from Jenkins](migration/jenkins.md)
 
 You can also get started by using one of the
 [`.gitlab-ci.yml` templates](https://gitlab.com/gitlab-org/gitlab-foss/tree/master/lib/gitlab/ci/templates)

doc/ci/migration/circleci.md

 ---
+stage: Verify
+group: Continuous Integration
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
 comments: false
 type: index, howto
 ---