Fix sidebar grouping: source files get their own "Source" accordion

Source file entries (e.g. compartments.yaml) were falling through to
the default "Types" group label which made no sense. Add explicit
"source" case to kindGroupLabel.

Author: daniel-garcia

internal/site/static/assets/app.js

@@ -719,6 +719,7 @@
       case "record": return "Records";
       case "jsonschema": return "Schemas";
       case "parquet": return "Columns";
+      case "source": return "Source";
       default: return "Types";
     }
   }