[Doc][CI/Build] Update docs and tests to use vllm serve (#6431)

examples/openai_vision_api_client.py

@@ -1,9 +1,7 @@
 """An example showing how to use vLLM to serve VLMs.
 
 Launch the vLLM server with the following command:
-python -m vllm.entrypoints.openai.api_server \
-    --model llava-hf/llava-1.5-7b-hf \
-    --chat-template template_llava.jinja
+vllm serve llava-hf/llava-1.5-7b-hf --chat-template template_llava.jinja
 """
 import base64