autocompletion should list stages by appearance order in the tutorial

[lavocatt]

the autocompletion returns the list of stages to break at sorted by alphabetical order:

└──╼> markdown-runner docs/tutorials/prometheus_locked_down.md --break-at 
certs       deploy      grafana     init        messaging   scrape      teardown    test_setup  verify 

Which is not practical to know where to stop if you're aiming for the previous to the last one for instance.