mermaid graphs should ends with .mmd extension

Following https://mermaid.js.org/ecosystem/integrations-create.html

src/outils_graph.py

@@ -73,7 +73,7 @@ def generate_graph_mermaid(filename, dico, label_node = True, label_edge = True,
         return txt
     txt = get_graph_wo_operations_mermaid_temp(dico, txt, 0)
 
-    with open(f"{filename}.md", "w") as text_file:
+    with open(f"{filename}.mmd", "w") as text_file:
         text_file.write(txt)