__vars__ in documentation

### Search for existing feature requests

- [X] I have searched the issue tracker and confirmed that I was unable to find existing issues for the feature I am requesting.

### The feature request

Please provide some pointers in the documentation and/or the config variables page about how variables can be used in code.

### Proposed solution

```python
print(__vars__['var_name'])
```

### Additional context

_No response_

先從現有文件以及 issue 中提到的設定變數頁面開始，然後檢查目前如何將變數公開給 Python 程式碼。記錄如何使用提議的 __vars__['var_name'] 形式存取變數，並在使用者能夠在相關文件中找到清楚的使用說明時，視為工作完成。

由索引模型根據 Issue 內容生成。