Update documentation to explain the use of OpenAI API compatible models using OpenRouter

[srdas]

For models that are compatible with the OpenAI library Jupyter AI provides configuration via OpenRouter.
By supporting the configuration of parameters such as api_key, base_url, and model, various large model services compatible with the OpenAI library call methods can be used.

While the functionality exists, it is missing in the documentation, which needs to be updated.
Suggested location in the documents is here: https://jupyter-ai.readthedocs.io/en/latest/users/index.html#model-providers

For example, Deepseek may be used via OpenRouter as shown below. Specify the base url and model, and enter the API key in the OpenRouter slot for the API key:

On testing we will see: