docs: clarify Realtime API createHeaders examples and correct authentication usage

[naresh4dev]

• Updates the createHeaders examples to avoid confusion where two providers were shown or the usage was ambiguous. The current docs reference conflicting provider usage in createHeaders and do not clearly explain how Portkey authentication should be implemented for Realtime API calls
• Adds correct guidance around Portkey API key vs OpenAI key usage, including explicitly stating that api_key="dummy" is not required in the published examples, and that the Portkey API key should be passed instead. This fixes an omission where the docs did not show the recommended header pattern used in other integration pages.
• Clarifies that Realtime API users should authenticate using Portkey credentials via createHeaders(provider, apiKey) rather than incorrectly passing OpenAI API key into the client’s api_key property when routing through Portkey.

[siddharthsambharia-portkey]

I'm not sure if this will work afaik API key is probably needed in the async open AI client.

Also, now with Portkey's new model catalog, you actually do not need the create headers method. You can just directly use the async open AI client and then pass the model in the model field with the rate provider slug/model ID.

[vrushankportkey]

on api_key - send PORKTEY_API_KEY

No need to send provider header as well. Just send provider slug as part of the model param.