[Doc] Add environment variables configuration reference (#6217)

Signed-off-by: Hongsheng Liu <liuhongsheng4@huawei.com>

docs/configuration/README.md

@@ -7,6 +7,9 @@ For options within a vLLM Engine, please refer to the [vLLM 0.26 configuration g
 Each model defines fixed topology in a registered `PipelineConfig` and runtime
 overrides in a deploy YAML.
 
+For process-level settings shared by the CLI, deploy YAML, workers, and model
+integrations, see [Environment Variables](./environment_variables.md).
+
 For a specific example, see the [Qwen2.5-Omni deploy config](gh-file:vllm_omni/deploy/qwen2_5_omni.yaml). The matching frozen pipeline topology lives at [vllm_omni/model_executor/models/qwen2_5_omni/pipeline.py](gh-file:vllm_omni/model_executor/models/qwen2_5_omni/pipeline.py).
 
 For an introduction, see [Pipeline and deploy configurations](./stage_configs.md).