Improve readme, that entra-ID connections in AI studio are not supported

### This issue is for a: (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [ ] feature request
- [x] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
set up the connections to openAI + AI search in AI Hub via managed identity and run the build_index script

### Any log messages given by the failure
Failed to upload documents to IndexStoreType.ACS index due to: () Authorization failed.

### Expected/desired behavior
upload works

### Mention any other details that might be useful
I had the connections defined in AI Studio with entraID authentication (correct roles set and everything working). As soon as I switched to key-based authentication the script worked as desired.

先阅读 README 和 build_index 脚本，然后查看 issue 中描述的、已记录的 AI Studio 连接设置。更新 README，说明不支持经过 Entra ID 身份验证的连接，并说明受支持的基于密钥的身份验证要求。当用户能够理解此限制并据此配置示例时，即视为完成。

由索引模型根据 Issue 内容生成。