fix: demo 页面优化

[xingwanying]

• demo页面视图切换卡片垂直居中对齐
• 将demo页面结构从Article中抽出
• 修改AVAheader文案

[github-actions[bot]]

🎊 PR Preview 5b2a8c7910fc3534a2c94632eba274e41c6ca076 has been successfully built and deployed to https://antvis-gatsby-theme-antv-preview-pr-271.surge.sh

:clock1: Build time: 327.944s

_{🤖 By surge-preview}

[afc163]

这里文字和 tabs 贴的好近，另外左边距是不是要和 API 对齐？

[afc163]

收起所有不垂直居中。

[afc163]

标题栏不应该放在滚动区域里，应该始终展现。