Create a GitLab-based demo repo alongside the GitHub version

[apirogov]

The demo repo for GitLab similarly should serve two purposes:

• showing how the result will look like
• running the CI pipeline on GitLab and making sure that everything works (e.g. using this)

open question is which GitLab instance it should be hosted on?

[apirogov]

this could be useful https://github.com/jakob-fritz/github2lab_action