[Doc] Add environment variables doc (#519)

### What this PR does / why we need it?
Add environment variables doc.
---------

Signed-off-by: shen-shanshan <467638484@qq.com>

docs/source/index.md

@@ -45,6 +45,7 @@ faqs
 :maxdepth: 1
 user_guide/suppoted_features
 user_guide/supported_models
+user_guide/env_vars
 user_guide/release_notes
 :::
 

docs/source/user_guide/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
+:::