[GLUTEN-2139][DOC] Document Gluten UI

docs/get-started/Velox.md

@@ -595,3 +595,34 @@ Both Parquet and ORC datasets are sf1024.
 # External reference setup
 
 TO ease your first-hand experience of using Gluten, we have set up an external reference cluster. If you are interested, please contact Weiting.Chen@intel.com.
+
+# Gluten UI
+
+## Gluten event
+
+Gluten provides two events `GlutenBuildInfoEvent` and `GlutenPlanFallbackEvent`:
+
+- GlutenBuildInfoEvent, it contains the Gluten build information so that we are able to be aware of the environment when doing some debug.
+  It includes `Java Version`, `Scala Version`, `GCC Version`, `Gluten Version`, `Spark Version`, `Hadoop Version`, `Gluten Revision`, `Backend`, `Backend Revision`, etc.
+
+- GlutenPlanFallbackEvent, it contains the fallback information for each query execution.
+  Note, if the query execution is in AQE, then Gluten will post it for each stage.
+
+Developers can register `SparkListener` to handle these two Gluten events.
+
+## SQL tab
+
+Gluten provides a tab based on Spark UI, named `Gluten SQL / DataFrame`
+
+![Gluten-UI](../image/gluten-ui.png)
+
+This tab contains two parts:
+
+1. The Gluten build information.
+2. SQL/Dataframe queries fallback information.
+
+If you want to disable Gluten UI, add a config when submitting `--conf spark.gluten.ui.enabled=false`.
+
+## History server
+
+Gluten UI also supports Spark history server. Add gluten-ui jar into the history server classpath, e.g., $SPARK_HOME/jars, then restart history server.