Rework CI/CD excercises to make them simpler

[theFeu]

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At this point the excercises aren't as straightforwards as I would like them to be. There are a bunch of commands that are a little too complex to expect people to know about them and I would rather not have to explain python package managing in a Git / GitLab training...

Also we're kind of dependant on external packages if we continue here. Maybe we want to change that too, see #244