add citydoctor repo link

public/content/content.json

@@ -605,6 +605,7 @@
     "links": {
       "publication": "https://doi.org/10.1007/s41064-020-00094-0",
       "project": "https://www.hft-stuttgart.de/forschung/projekte/aktuell/citydoctor-2"
+      "repo": "https://transfer.hft-stuttgart.de/gitlab/citydoctor/citydoctor2"
     },
     "refIndex": 7
   },