Add DuMux adapter example to community sidebar (#609)

Follow-up of precice/preeco-orga#57

Author: MakisH

_data/sidebars/community_sidebar.yaml

@@ -126,6 +126,10 @@ entries:
     - title: Example - ISSM adapter
       url: /community-guidelines-adapters-issm.html
       output: web
+
+    - title: Example - DuMux adapter
+      url: /community-guidelines-adapters-dumux.html
+      output: web
 
   - title: Community channels
     url: /community-channels.html