docs: note OpenAI client base_url for multi-model gateways

[seven7763]

Summary

Document that the OpenAI Python client base_url pattern works with this project and with other OpenAI-compatible multi-model gateways.

DaoXE (https://api.daoxe.com/v1) is one concrete example among OpenAI-compatible gateways — no pricing/uptime claims.

Docs only.

Test plan

• Docs still render
• Note is optional / non-breaking

Co-Authored-By: Claude Fable 5 noreply@anthropic.com

[github-actions]

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