docs: add FuturMix.ai as documented OpenAI-compatible provider

[FuturMix]

Summary

Adds documentation for using FuturMix.ai as an OpenAI-compatible provider in the "Using different models" guide, following the same pattern as Google Gemini and OpenRouter.

FuturMix is a unified AI gateway providing access to 22+ models (Claude, GPT, Gemini) through a single OpenAI-compatible endpoint.

Changes

• Added "Using FuturMix.ai Models" section to docs/source/en/examples/using_different_models.md
• Follows existing documentation pattern (OpenAIModel with api_base)
• Includes examples for switching between model families

Example

from smolagents import OpenAIModel

model = OpenAIModel(
    model_id="claude-sonnet-4-20250514",
    api_base="https://futurmix.ai/v1",
    api_key=FUTURMIX_API_KEY,
)

No code changes required — uses existing OpenAIModel class.