Add written documentation showing a Jenkins migration

Problem to solve

We have Jenkins migration docs at https://docs.gitlab.com/ee/ci/migration/jenkins.html but it's structured more as a reference. It would also be valuable to have something in written form. We should do this after delivering the wrapper MVC (&2779) and include that.

Further details

Proposal

A video is a good step towards helping show people how easy it is to migrate from Jenkins to GitLab. We can walk people through a basic but complete Jenkins installation and how to migrate that over using tools and concepts available today.

Who can address the issue

Other links/references