[DOC] add config option spark.ui.enabled into document

## What changes were proposed in this pull request?

The configuration doc lost the config option `spark.ui.enabled` (default value is `true`)
I think this option is important because many cases we would like to turn it off.
so I add it.

## How was this patch tested?

N/A

Author: WeichenXu <WeichenXu123@outlook.com>

Closes #14604 from WeichenXu123/add_doc_param_spark_ui_enabled.

docs/configuration.md

@@ -589,6 +589,13 @@ Apart from these, the following properties are also available, and may be useful
     finished.
   </td>
 </tr>
+<tr>
+  <td><code>spark.ui.enabled</code></td>
+  <td>true</td>
+  <td>
+    Whether to run the web UI for the Spark application.
+  </td>
+</tr>
 <tr>
   <td><code>spark.ui.killEnabled</code></td>
   <td>true</td>