docs(configuration): document openai-responses protocol

[zhouzhih]

Summary

This PR updates the custom provider configuration docs to document the openai-responses protocol supported by OCR.

Fixes #556.

Changes

• Lists anthropic, openai, and openai-responses as valid custom provider protocol values.
• Adds an OpenAI Responses API (/v1/responses) custom provider example.
• Keeps the English, Chinese, and Japanese configuration docs consistent.

Non-goals

• This does not change provider resolution or LLM client behavior.
• This does not add or modify built-in providers.

Validation

• git diff --check origin/main...HEAD
• cd pages && npm run typecheck
• cd pages && npm run build

[github-actions]

✅ OpenCodeReview: No supported files changed.

[zhouzhih]

@lizhengfeng101 Could you please review this PR when you have time?

It fixes #556 by documenting the supported openai-responses custom provider protocol across the English, Chinese, and Japanese configuration guides.

[lizhengfeng101]

LGTM