add FAQ doc under 'serving' (vllm-project#5946)

docs/source/index.rst

@@ -84,6 +84,7 @@ Documentation
    serving/usage_stats
    serving/integrations
    serving/tensorizer
+   serving/faq
 
 .. toctree::
    :maxdepth: 1

docs/source/serving/faq.rst

@@ -0,0 +1,12 @@
+Frequently Asked Questions
+========================
+
+    Q: How can I serve multiple models on a single port using the OpenAI API?
+
+A: Assuming that you're referring to using OpenAI compatible server to serve multiple models at once, that is not currently supported, you can run multiple instances of the server (each serving a different model) at the same time, and have another layer to route the incoming request to the correct server accordingly.
+
+----------------------------------------
+
+    Q: Which model to use for offline inference embedding?
+
+A: If you want to use an embedding model, try: https://huggingface.co/intfloat/e5-mistral-7b-instruct. Instead models, such as Llama-3-8b, Mistral-7B-Instruct-v0.3, are generation models rather than an embedding model