Add custom ports to documentation

Summary:
Added instructions on using custom ports.

We probably want to put the headless instructions here too when they're ready.

Reviewed By: danielbuechele

Differential Revision: D13860469

fbshipit-source-id: 76537a36e6fc85e805a29830927e4d59ee4f73e8

website/sidebars.json

@@ -29,6 +29,7 @@
       "error-handling",
       "testing"
     ],
-    "Flipper Internals": ["establishing-a-connection"]
+    "Flipper Internals": ["establishing-a-connection"],
+    "Advanced Usage": ["custom-ports"]
   }
 }