Add Tutorial for GitLab CI

[SimeonEhrig]

The tutorial provide an example CI pipeline for GitLab CI with the following jobs

1. Set up search criteria for depending packages in an example project.
2. Execute the dependency graph search algorithm.
3. Generate example integration jobs.
4. Run the example integration jobs.

Generating and running the integration jobs is GitLab CI specific and done with downstream pipelines: https://docs.gitlab.com/ee/ci/pipelines/downstream_pipelines.html