Add GitLab CI vs. Jenkins to comparison page

PROS:

• Version controlled configuration in the form of .gitlab-ci.yml means:
  1. You know how a commit was tested
  2. People can easily learn the configuration from other projects
  3. New features (branches) can change the testing
  4. When pushing an old branch it uses the old tests
  5. You don't need to find your 'build person' to change the configuration
• Closely integrated into the GitLab UI so you can quickly check the build results.
• No need to do any setup for a new project.
• GitLab prevents you from executing tests on the CI machine. You need to run a separate process to encourage this best practise.
• Jenkins can become a plugin hell, with GitLab it's simple
• From https://blog.trainline.eu/12703-building-on-gitlab-ci:
  • Reliable dockerized builds
  • it scales
  • Easier to control
  • Tests on merge requests
  • Better UI

CONS:

• Jenkins has a huge ecosystem with a lot of plugins and integrations
• Jenkins has a UI for configuration

PLEASE ADD MORE

/cc @ayufan @grzesiek @tmaczukin @Haydn