Add info for federation links

Reported via public slack channel:

https://rabbitmq.slack.com/archives/C1EDN83PA/p1586983265300300

deps/rabbitmq_management/priv/www/api/index.html

@@ -1012,6 +1012,17 @@ or:
           <pre>curl -4u 'guest:guest' -XPOST localhost:15672/api/rebalance/queues/</pre>
         </td>
       </tr>
+      <tr>
+        <td>X</td>
+        <td></td>
+        <td></td>
+        <td></td>
+        <td class="path">/api/federation-links<br/>
+                         /api/federation-links/<i>vhost</i></td>
+        <td>
+          Provides status for all federation links. Requires the <code>rabbitmq_federation_management</code> plugin to be enabled.
+        </td>
+      </tr>
     </table>