docs: add MiniMax as an OpenAI-compatible provider example

[octo-patch]

Summary

• Add MiniMax as an OpenAI-compatible provider example in README and documentation
• MiniMax provides an OpenAI-compatible API at https://api.minimax.io/v1, so it works seamlessly with the existing OpenAIModel class
• Added usage examples following the same pattern as existing providers (Google Gemini, OpenRouter, Together AI)

Changes

• README.md: Added a new collapsible section for MiniMax under the OpenAI-compatible servers examples
• docs/source/en/examples/using_different_models.md: Added a dedicated "Using MiniMax Models" section with setup instructions and code examples

Test plan

• Verified MiniMax API is OpenAI-compatible and works with OpenAIModel
• Documentation follows existing patterns (Gemini, OpenRouter sections)
• No code changes required - only documentation additions