[Doc] Update user doc index (#1581)

Add user doc index to make the user guide more clear
- vLLM version: v0.9.1
- vLLM main:
49e8c7ea25

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

docs/source/user_guide/configuration/env_vars.md

@@ -0,0 +1,9 @@
+# Environment Variables
+
+vllm-ascend uses the following environment variables to configure the system:
+
+:::{literalinclude} ../../../../vllm_ascend/envs.py
+:language: python
+:start-after: begin-env-vars-definition
+:end-before: end-env-vars-definition
+:::