gitlab-ci: trigger documentation pipeline on doc changes

Michael Krausch-Hofmannrequested to merge
mh21_rh/containers:ci-documentation-mr-trigger into main

Test documentation changes in MRs by triggering the documentation repository pipeline, similar to how cki-lib does it.

This ensures that markdown changes in the containers repo are validated against the documentation build before merging.

Part of #105 (closed)

Edited by Michael Krausch-Hofmann

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