docs(faq): explain empty workflow model selection

[zgzyq]

Description

• explain that models must first be added in the model management interface
• mirror the FAQ entry in English and Chinese
• base the answer on the resolved setup question in [GENERAL] Model Selection Dropdown Is Empty in Workflow Creation (Create → Workflow → LLM) After Minimal Deployment #800

Part of #1412.

Testing

• cd docs && npm run docs:build
• VitePress build completed successfully

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[chatgpt-codex-connector]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-01T06:34:38.468227Z	1c81b6f	PR opened

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[FenjuFu]

文档内容已核对，符合 #1412，并准确反映 #800 的解决方案；英中 FAQ 条目位置和语义一致。当前还有两个合并阻塞项：提交 1c81b6f 的 DCO 为 ACTION_REQUIRED，请为该提交补充匹配作者身份的 Signed-off-by；CLA 也仍为 pending，请完成签署。两项门禁通过后可继续 approve。