chore: add css variables content for api docs

[anlyyao]

🤔 这个 PR 的性质是？

• [ ] 日常 bug 修复
• [ ] 新特性提交
• [ ] 文档改进
• [ ] 演示代码改进
• [ ] 组件样式/交互改进
• [ ] CI/CD 改进
• [ ] 重构
• [ ] 代码风格优化
• [ ] 测试用例
• [ ] 分支合并
• [ ] 其他

🔗 相关 Issue

相关 PR

common: https://github.com/Tencent/tdesign-common/pull/1527 https://github.com/Tencent/tdesign-common/pull/1528

其他说明

form、list、table、swipe-cell 组件未抽离变量，sticky 组件无css variable

💡 需求背景和解决方案

📝 更新日志

• site: API文档新增 css variables 内容

• [x] 本条 PR 不需要纳入 Changelog

☑️ 请求合并前的自查清单

⚠️ 请自检并全部勾选全部选项。⚠️

• [ ] 文档已补充或无须补充
• [ ] 代码演示已提供或无须提供
• [ ] TypeScript 定义已补充或无须补充
• [ ] Changelog 已提供或无须提供

[github-actions[bot]]

Size Change: 0 B

Total Size: 387 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/reset.css` | 672 B | | `./dist/tdesign.css` | 37.4 kB | | `./dist/tdesign.js` | 204 kB | | `./dist/tdesign.min.css` | 27.1 kB | | `./dist/tdesign.min.js` | 118 kB |

[github-actions[bot]]

[github-actions[bot]]

Size Change: 0 B

Total Size: 387 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/reset.css` | 672 B | | `./dist/tdesign.css` | 37.4 kB | | `./dist/tdesign.js` | 204 kB | | `./dist/tdesign.min.css` | 27.1 kB | | `./dist/tdesign.min.js` | 118 kB |

[github-actions[bot]]

Size Change: 0 B

Total Size: 387 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/reset.css` | 672 B | | `./dist/tdesign.css` | 37.5 kB | | `./dist/tdesign.js` | 204 kB | | `./dist/tdesign.min.css` | 27.2 kB | | `./dist/tdesign.min.js` | 118 kB |

[github-actions[bot]]

Size Change: 0 B

Total Size: 387 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/reset.css` | 672 B | | `./dist/tdesign.css` | 37.5 kB | | `./dist/tdesign.js` | 204 kB | | `./dist/tdesign.min.css` | 27.2 kB | | `./dist/tdesign.min.js` | 118 kB |

[github-actions[bot]]

Size Change: 0 B

Total Size: 387 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/reset.css` | 672 B | | `./dist/tdesign.css` | 37.5 kB | | `./dist/tdesign.js` | 204 kB | | `./dist/tdesign.min.css` | 27.2 kB | | `./dist/tdesign.min.js` | 118 kB |

[github-actions[bot]]

Size Change: 0 B

Total Size: 387 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/reset.css` | 672 B | | `./dist/tdesign.css` | 37.5 kB | | `./dist/tdesign.js` | 204 kB | | `./dist/tdesign.min.css` | 27.2 kB | | `./dist/tdesign.min.js` | 118 kB |

[github-actions[bot]]

Size Change: 0 B

Total Size: 387 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/reset.css` | 672 B | | `./dist/tdesign.css` | 37.5 kB | | `./dist/tdesign.js` | 204 kB | | `./dist/tdesign.min.css` | 27.2 kB | | `./dist/tdesign.min.js` | 118 kB |

[github-actions[bot]]

Size Change: 0 B

Total Size: 387 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/reset.css` | 672 B | | `./dist/tdesign.css` | 37.5 kB | | `./dist/tdesign.js` | 204 kB | | `./dist/tdesign.min.css` | 27.2 kB | | `./dist/tdesign.min.js` | 118 kB |

[github-actions[bot]]

Size Change: 0 B

Total Size: 387 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/reset.css` | 672 B | | `./dist/tdesign.css` | 37.5 kB | | `./dist/tdesign.js` | 204 kB | | `./dist/tdesign.min.css` | 27.2 kB | | `./dist/tdesign.min.js` | 118 kB |

[github-actions[bot]]

Size Change: 0 B

Total Size: 387 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/reset.css` | 672 B | | `./dist/tdesign.css` | 37.5 kB | | `./dist/tdesign.js` | 204 kB | | `./dist/tdesign.min.css` | 27.2 kB | | `./dist/tdesign.min.js` | 118 kB |

[github-actions[bot]]

Size Change: 0 B

Total Size: 387 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/reset.css` | 672 B | | `./dist/tdesign.css` | 37.5 kB | | `./dist/tdesign.js` | 204 kB | | `./dist/tdesign.min.css` | 27.2 kB | | `./dist/tdesign.min.js` | 118 kB |