beautify documentation

Signed-off-by: Silvano Cirujano Cuesta <silvano.cirujano-cuesta@siemens.com>

Author: Silvanoc

data-model/resources/markdown-templates/index.md.jinja2

@@ -35,6 +35,8 @@ These are the classes directly participating in Margo interfaces (e.g. APIs):
 
 ??? note "Complete Margo data model"
 
+    ## Detailed Class Diagram
+
     This is a class diagram showing all the classes involved in the Margo data model.
     Use the mouse wheel to zoom and click-drag to pan. The controls in the bottom-right corner allow zooming in, resetting, and zooming out.