Add onboarding documentation for specific areas

Talking with a developer that is trying GitLab CI/CD, the complain was that there is no simple way to go from scratch to a fully functional environment.

There are a lot of "options" to choose from (different ways of GitLab installations for example), but no best practices or simple flows that will bring you to a complete instance.

We should create documentation that can cover at least this path:

• Install GitLab instance
• Install GitLab Runner
• Configure the runner
• Install and configure Prometheus (?)

No assumption about previous knowledge should be done (e.g., docker, k8s).

What they asked for is: don't give me options where I need to study something else before I can reach the goal, just say what I need to do.

It is something similar to what we are doing with onboarding features (auto devops, gke integration). Let's try to do the same for documentation!