add demo screenshots (#3)

Author: ksqsf

demo/index.md

@@ -0,0 +1,19 @@
+# 展示
+
+## 多彩皮肤
+
+![截图演示](/demo.png)
+
+<img src="/demo/background-layout-font.png" width="350">
+
+（背景图：[Close Up Photography of White and Black Cat](https://www.pexels.com/photo/close-up-photography-of-white-and-black-cat-69932/), CC0 许可）
+
+## 卷轴模式
+
+![卷轴模式](/demo/win11-light-scroll.png)
+
+## 深度定制
+
+[定制 CSS](../docs/theme/css.md) 可以深度个性化界面，异形窗口等也可实现：
+
+![异形候选窗](/demo/decoration.png)

index.md

@@ -10,6 +10,9 @@ hero:
     - theme: brand
       text: 安装
       link: https://github.com/fcitx-contrib/fcitx5-macos-installer/blob/master/README.zh-CN.md
+    - theme: alt
+      text: 展示
+      link: /demo/
     - theme: alt
       text: 文档
       link: /docs/