docs: show OpenAI-compatible gateway setup

[cerebrixos]

Summary

• add an OpenAI-compatible gateway example to the OpenAI provider page
• show how to route Prism through Tuning Engines by changing the provider URL and API key
• keep Prism application calls unchanged with ->using('openai', 'gpt-4o')

Why

We are a Rails and AI infrastructure team using Prism with Tuning Engines. Since Prism already supports configurable provider URLs, this gives Laravel/PHP teams a simple setup path for governed model access, policy checks, traces, audit logs, and usage/cost reporting without changing their Prism application logic. It would mean a lot to us if you are open to including it, and I am happy to revise the copy to fit the docs style.

Validation

• git diff --check

[cerebrixos]

Hello team @pocketarc . Please do consider this PR for merge. So much appreciate in advance :)