Update navigation.yml

_data/navigation.yml

 main:
-  - title: "Blog"
-    url: https://blog.funkwhale.audio/local
   - title: "Demo"
     url: https://demo.funkwhale.audio
+  - title: "Blog"
+    url: https://blog.funkwhale.audio/local
+  - title: "Forums"
+    url: https://socialhub.network/c/projects/funkwhale
   - title: "Source"
     url: https://code.eliotberriot.com/funkwhale/funkwhale
   - title: "Documentation"