docs: add Tuning Engines LiteLLM config

[cerebrixos]

Description

Adds a documentation-only example for routing RD-Agent chat and embedding calls through Tuning Engines using the existing LiteLLM/OpenAI-compatible configuration.

Motivation and Context

RD-Agent already documents unified and separate LiteLLM API bases. This adds a concrete OpenAI-compatible gateway example so users can configure Tuning Engines with OPENAI_API_BASE and a gateway-issued inference key.

How Has This Been Tested?

• If you are adding a new feature, test on your own test scripts.

Docs-only change; no runtime tests were run.

Screenshots of Test Results (if appropriate):

N/A

Types of changes

• Fix bugs
• Add new feature
• Update documentation

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📚 Documentation preview 📚: https://RDAgent--1419.org.readthedocs.build/en/1419/

[cerebrixos]

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