[GLUTEN-11524][VL][DOC] Update gpu documentation

[marin-ma]

Update gpu documentation regarding dynamic execution, hybrid execution, gpu concurrency and async shuffle read.

Related issue: #11524

[Copilot]

Pull request overview

Updates VeloxGPU.md to expand the GPU documentation around stage-level execution mode selection, hybrid CPU/GPU scheduling concepts, GPU concurrency, and async shuffle read tuning for the Velox backend.

Changes:

• Reworks “Dynamic Execution” to describe AQE-stage execution-mode adjustment.
• Adds new sections for “CPU/GPU Hybrid Execution” and performance tuning (GPU concurrency + async shuffle reader).
• Renumbers downstream sections to account for the new content.

Suppressed comments (2)

docs/get-started/VeloxGPU.md:114

• The properties snippet uses spark.gluten.sql.columnar.hybridExecution.* settings that do not exist in the codebase (they only appear in this doc). This will fail for users who copy/paste the config.

spark.gluten.auto.adjustStageResource.enabled = true
spark.gluten.sql.columnar.hybridExecution.enabled = true

# Step 3 – tell Spark about the GPU resource on each executor
spark.gluten.sql.columnar.hybridExecution.gpuResource.amountPerTask = 0.1 # fractional: 10 concurrent GPU tasks/executor

docs/get-started/VeloxGPU.md:127

• These table rows document spark.gluten.sql.columnar.hybridExecution.* and spark.gluten.sql.columnar.gpu.onlyOffloadJoinStage, but neither key exists anywhere in the codebase (only in this markdown). The table should avoid listing non-existent configuration keys.

| `spark.gluten.sql.columnar.hybridExecution.enabled` | `true` | Enable CPU/GPU hybrid execution. Stages are scheduled to CPU or GPU nodes based on their execution mode. Requires AQE (`spark.sql.adaptive.enabled=true`). |
| `spark.gluten.sql.columnar.hybridExecution.gpuResource.name` | `gpu` | The Spark custom-resource name for GPU. Must match `spark.executor.resource.<name>.*` and `spark.task.resource.<name>.*`. |
| `spark.gluten.sql.columnar.hybridExecution.cpuResource.name` | `cpu` | The Spark custom-resource name for CPU. Must match `spark.executor.resource.<name>.*` and `spark.task.resource.<name>.*` for CPU-stage scheduling to take effect. |
| `spark.gluten.sql.columnar.hybridExecution.gpuResource.amountPerTask` | `0.1` | Fractional GPU resource amount per task. Controls how many GPU tasks can run concurrently on a single executor (e.g. `0.1` → 10 tasks share 1 GPU). |
| `spark.gluten.sql.columnar.gpu.onlyOffloadJoinStage` | `true` | When `true`, only stages that contain a join operator are offloaded to GPU. All other stages execute on CPU. Useful for workloads where only join-heavy stages benefit from GPU acceleration. |

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