docs: fix Export plugin docs and refresh demo

[louhaojie99]

📝 Description

对照 src/plugin/export 源码，修正 Export 插件官方教程文档，并重写 examples 示例页。

修改文件

路径	说明
site/docs/tutorial/plugins/export.zh.md	中文教程
site/docs/tutorial/plugins/export.en.md	英文教程
examples/src/pages/plugins/export/index.tsx	示例逻辑与交互
examples/src/pages/plugins/export/index.less	左画布 + 右操作面板布局

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一、文档（site/docs/tutorial/plugins/）

1. 选项类型名（与 type.ts 一致）

   • Export.ToSVGOptions → ExportToSVGOptions
   • Export.ToImageOptions → ExportToImageOptions
   • 涉及 exportSVG、exportPNG、exportJPEG、toSVG、toPNG、toJPEG 的签名与正文

2. Graph 方法签名（与 api.ts 一致）

   • toSVG、toPNG、toJPEG 的回调类型改为 ExportToSVGCallback（不再使用内联 (dataUri: string) => any 等写法）
   • 补充 toSVGAsync、toPNGAsync、toJPEGAsync 的 Promise<string> 签名

3. 选项表

   • 「类型」列统一使用反引号（如 `RectangleLike`、`boolean`、`NumberExt.SideOptions`）
   • backgroundColor 默认值改为 'white'（与 options.backgroundColor || 'white' 一致）

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二、Examples（examples/src/pages/plugins/export/）

1. 布局：左侧 800×600 画布，右侧分组操作面板（index.less）
2. 交互：按三组 API 分列按钮——exportSVG / exportPNG / exportJPEG、toSVG / toPNG / toJPEG、toSVGAsync / toPNGAsync / toJPEGAsync
3. 反馈：message 提示改为中文；toSVG 保存 SVG 文件，toPNG / toJPEG 及其 Async 在新标签页预览 Data URI
4. 代码：去掉泛型 Pick + graph[type] 写法，改为显式 handler；补充文件头与分组注释（方法名写全，不用通配符缩写）

🖼️ Screenshot

修改前

修改后

💡 Motivation and Context

官方教程中 Export 插件的选项类型、方法签名与 src/plugin/export 不一致，开发者按文档编写 TypeScript 容易出错；同时 examples 示例完善的更易理解的交互示例。

🧩 Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Enhancement (changes that improvement of current feature or performance)
• Refactoring (changes that neither fixes a bug nor adds a feature)
• Test Case (changes that add missing tests or correct existing tests)
• Code style optimization (changes that do not affect the meaning of the code)
• Docs (changes that only update documentation)
• Chore (changes that don't modify src or test files)

🔍 Self Check before Merge

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[gemini-code-assist]

Code Review

This pull request refactors the Export plugin example by introducing specific handlers for downloading, callback-based exporting, and asynchronous exporting of SVG, PNG, and JPEG formats, alongside updated documentation and styling. Feedback on these changes highlights a potential XSS vulnerability in the image preview function due to the use of innerHTML, and a logic issue where success messages are displayed even if the graph reference is null.

[louhaojie99]

@hustcc