docs: add MiniMax reference-image request example

[octo-patch]

Reason: Document the native MiniMax reference-image request and response flow.

• Add the global and mainland China image generation endpoints.
• Show the subject_reference object required for character consistency.
• Document supported models, output handling, completion metadata, and URL expiry.

Checks:

• npx prettier --check --range-start 21035 --range-end 22435 docs/en/stage-1/integrating-ai-capabilities/index.md
• git diff --check