[Doc] Add instructions for using Ollama with AI Gateway

[zhengkezhou1]

Why need this?

For people facing geographic restrictions (unable to directly access the OpenAI platform) but who still wish to experience AI Gateway, providing a guide on how to use Ollama would be extremely helpful. This is because I spent a long time debugging before successfully using Ollama as an LLM provider.

[teaglebuilt]

i think there is documentation that explains how to use self hosted options.