Add video on performing a Jenkins migration

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Problem to solve

We have Jenkins migration docs at https://docs.gitlab.com/ee/ci/jenkins/index.html but it's structured more as a video that someone could sit down and watch. We should do this as after completing the wrapper MVC (&2779) and use that as part of the video.

Further details

Proposal

A blog post 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

Edited by 🤖 GitLab Bot 🤖