Document diverged OpenAI API

Nobody has claimed this yet.

The existence of the diverged OpenAI API appears to be undocumented at the moment, but has some useful and requested features (e.g. `repetition_penalty` #1914).

As far as I can tell, it appears these endpoints can be used with the OpenAI client, and additional parameters passed with the `extra_body` argument, e.g.:

```python
# ...
openai.base_url = "http://localhost:8000/api/v1/"
# ...
completion = openai.chat.completions.create(
model=model,
messages=[{"role": "user", "content": "Hello! What is your name?"}],
extra_body={
"repetition_penalty": 1.10
}
)
```

(this is also useful for `top_k` which is not a supported argument for the OpenAI client but is supported in both APIs in FastChat)

It would be good to at least document that this API exists and confirm whether the divergences actually impact using the OpenAI client.

Start by locating FastChat's OpenAI-compatible API documentation and serving entry points, then compare the documented endpoints with the OpenAI client example in the issue. Document that the diverged API exists and clarify whether extra_body supports parameters such as repetition_penalty and top_k.

Written by the indexing model from the issue text.