Adds link to ESO developer documentation in nav (elastic#187867)

## Summary

Adds link to the new encrypted saved objects developer documentation in
the left hand navigation under "key concepts".

nav-kibana-dev.docnav.json

@@ -80,6 +80,10 @@
           "id": "kibDevDocsSavedObjectsIntro",
           "label": "Saved objects"
         },
+        {
+          "id": "kibDevDocsEncryptedSavedObjectsIntro",
+          "label": "Encrypted Saved objects"
+        },
         {
           "id": "kibDevDocsPersistableStateIntro"
         },
@@ -614,4 +618,4 @@
       ]
     }
   ]
-}
+}