[DOC] Add doc about experimental feature using off-heap to store broadcast build relation (#8882)

Author: zjuwangg

docs/Configuration.md

@@ -98,6 +98,7 @@ The following configurations are related to Velox settings.
 | spark.gluten.velox.fs.s3a.connect.timeout                            | Timeout for AWS s3 connection.                                                                                                                     | 1s                |
 | spark.gluten.sql.columnar.backend.velox.orc.scan.enabled             | Enable velox orc scan. If disabled, vanilla spark orc scan will be used.                                                                           | true              |
 | spark.gluten.sql.complexType.scan.fallback.enabled                   | Force fallback for complex type scan, including struct, map, array.                                                                                | true              |
+| spark.gluten.velox.offHeapBroadcastBuildRelation.enabled             | Experimental: If enabled, broadcast build relation will use offheap memory. Otherwise, broadcast build relation will use onheap memory, default value is false |                   |
 
 Additionally, you can control the configurations of gluten at thread level by local property.
 

docs/get-started/Velox.md

@@ -545,6 +545,30 @@ I20231121 10:19:42.348845 90094332 WholeStageResultIterator.cc:220] Native Plan
       queuedWallNanos              sum: 2.00us, count: 1, min: 2.00us, max: 2.00us
 ```
 
+
+## Broadcast Build Relations to Off-Heap(Experimental)
+
+The experimental feature **Off-Heap Broadcast Build Relations** aims to mitigate out-of-memory (OOM) issues caused by heap memory consumption during broadcast operations. Detailed design
+can be found [here](https://docs.google.com/document/d/1eZNWPUEdiz2JPJfhyVn9hrk6SqJFRNzOMZm6u5Yredk/edit?tab=t.0)
+
+### Purpose & how it works
+- **Avoid OOM**: Prevent OOM errors when broadcasting large datasets.
+- **Reduce Heap Memory Usage**: Store broadcast build relations in Spark off-heap memory instead of on-heap memory
+
+### Configuration
+
+### Enable Off-Heap Broadcast
+To enable this feature, you can set the following Spark configuration:
+
+| Property                                                    | Default | Description                                                       |
+|-------------------------------------------------------------|---------|-------------------------------------------------------------------|
+| `spark.gluten.velox.offHeapBroadcastBuildRelation.enabled`  | `false` | Enable/disable off-heap storage for broadcast build relations.    |
+
+This feature has been tested through a series of tests, and we are collecting more feedback from users. If you have memory problem on broadcast build relations, please try this feature and give more feedbacks.
+
+**Note**: This feature will become the default behavior once stabilized. Stay tuned for updates!
+
+
 # Accelerators
 
 Please refer [HBM](VeloxHBM.md) [QAT](VeloxQAT.md) [IAA](VeloxIAA.md) for details