add citygml viewer

public/content/content.json

@@ -653,6 +653,30 @@
     },
     "refIndex": 6
   },
+  {
+    "item": {
+      "title": "CityGML Web Viewer",
+      "project": "CityDoctor2",
+      "author": {
+        "firstName": "Matthias",
+        "lastName": "Betz",
+        "hftURL": "https://www.hft-stuttgart.de/p/matthias-betz"
+      },
+      "project_year_start": "2019",
+      "project_year_end": "2021",
+      "project_is_on_going": true,
+      "imageLink": "img/citygml-online-viewer.jpg",
+      "keywords": ["CityGML"],
+      "description_en": "The online tools for visualizing CityGML files on the web",
+      "description_de": "Die Online-Tools zur Visualisierung von CityGML-Dateien im Web"
+    },
+    "links": {
+      "project": "https://transfer.hft-stuttgart.de/gitlab/citydoctor/qualityade",
+      "repo": "https://transfer.hft-stuttgart.de/gitlab/citydoctor/citygml-viewer",
+      "demo": "https://transfer.hft-stuttgart.de/pages/citydoctor/citygml-viewer/"
+    },
+    "refIndex": 6
+  },
   {
     "item": {
       "title": "CityDoctor2",