docs(configuration): document openai responses protocol

[FenjuFu]

Summary

• document openai-responses as the third supported custom-provider protocol
• add an OpenAI Responses API configuration example
• keep the English, Chinese, and Japanese configuration docs aligned

Why

The Responses API has been supported since v1.7.10, but the custom-provider docs still listed only anthropic and openai. This closes the documentation gap reported in #556.

Validation

• git diff --check
• verified the example against internal/llm/responses_client.go URL normalization
• local site build not run because pages/node_modules is not installed; the repository build script also uses Unix-style NODE_ENV=...

Closes #556.

Contributed as a member of the iFLYTEK Astron community.

[github-actions]

✅ OpenCodeReview: No supported files changed.

[lizhengfeng101]

Thanks for the contribution! However, #562 was merged first and already addresses #556 with the same changes (documenting openai-responses protocol across all three locale files).

Your PR does have one piece of extra value that #562 lacks: the note about url accepting either the API base URL or the full /responses endpoint. That's a useful clarification.

You have two options:

1. Close this PR — since the core documentation gap is already filled by docs(configuration): document openai-responses protocol #562.
2. Rebase onto main and submit a smaller follow-up — keeping only the incremental note about URL flexibility (the base-URL-or-full-endpoint clarification) on top of what docs(configuration): document openai-responses protocol #562 already added.

Either way is fine — up to you!

[lizhengfeng101]

@FenjuFu