Add a syntax/ graph parsing test to CI-pipeline
Our current documentation pipeline is not triggered on branches other than master and develop. However, GitLab seems to expect that the pipeline is passed by any branch before merging. This blocks the merge requests.
cf. https://git.rwth-aachen.de/nfdi4ing/metadata4ing/metadata4ing/-/merge_requests/157#note_1735549
cf. solution suggestion: https://git.rwth-aachen.de/nfdi4ing/metadata4ing/metadata4ing/-/merge_requests/157#note_1735598