Write up basic documentation on how to migrate from Jenkins to GitLab CI

Moving from Jenkins to GitLab CI is a fairly common process our customers go through, with great results.

We can help to improve this process, by writing up a page which specifically notes a few areas:

• The big things they need to think about
  • Shared/Private runners
  • Self-service
  • Potential for reduced cloud spend (elastic runners) and reduced maintenance
• Other major areas
  • Moving off of plugins
  • Samples or other tools to make moving off common plugins easier
  • any thing else?
• If you are stuck and can't find what you are looking for
  • Leave a comment on the doc?
  • Contact the PM?