[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.