docs(providers): clarify OpenAI-compatible configuration

[QIU-Guanzong]

Summary

• Clarify that custom OpenAI-compatible providers need an API-root base URL and the model ID accepted by the service.
• Document that non-empty OPENAI_BASE_URL and OPENAI_API_KEY override the provider fields for both openai_legacy and openai_responses.
• Keep the English and Chinese provider guides aligned.

Related to #2624: this documents the existing environment-variable behavior without adding a vendor-specific example.

Validation

• npm run build (from docs/)
• git diff --check

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[devin-ai-integration]

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