Sphinx config should not store custom class instances

No custom classes should be stored to the configuration.
Same reasoning as in https://github.com/eclipse-score/docs-as-code/issues/288.

```
WARNING: Unsupported type at 'needs.render_context.draw_interface': draw_full_interface - filtering out [needs_config_writer.unsupported_type]
WARNING: Unsupported type at 'needs.render_context.draw_module': draw_full_module - filtering out [needs_config_writer.unsupported_type]
WARNING: Unsupported type at 'needs.render_context.draw_component': draw_full_component - filtering out [needs_config_writer.unsupported_type]
WARNING: Unsupported type at 'needs.render_context.draw_feature': draw_full_feature - filtering out [needs_config_writer.unsupported_type]
```

首先阅读 issue 288，并跟踪警告中提到的 Sphinx 配置路径：needs.render_context.draw_interface、draw_module、draw_component 和 draw_feature。确定这些自定义类实例在哪里进入配置；当它们不再存储在那里且不支持类型警告消失时，即表示完成。

由索引模型根据 Issue 内容生成。