Added link to documentation

_data/navigation.yml

@@ -2,4 +2,6 @@ main:
   - title: "Demo"
     url: https://demo.funkwhale.audio
   - title: "Source"
-    url: https://code.eliotberriot.com/funkwhale
+    url: https://code.eliotberriot.com/funkwhale/funkwhale
+  - title: "Documentation"
+    url: https://docs.funkwhale.audio