docs(site): update site pages for v0.1.5 theme features

- Update version references to 0.1.5 across demo/docs/index
- Add instance-level theme APIs to docs.html
- Add external theme following, theme inheritance docs
- Update index.html feature description

site/index.html

@@ -482,7 +482,7 @@
           <svg width="22" height="22" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><circle cx="12" cy="12" r="5"/><line x1="12" y1="1" x2="12" y2="3"/><line x1="12" y1="21" x2="12" y2="23"/><line x1="4.22" y1="4.22" x2="5.64" y2="5.64"/><line x1="18.36" y1="18.36" x2="19.78" y2="19.78"/><line x1="1" y1="12" x2="3" y2="12"/><line x1="21" y1="12" x2="23" y2="12"/></svg>
         </div>
         <h3>主题与国际化</h3>
-        <p>基于 CSS 变量的主题系统，内置亮 / 暗 / 暖色三套主题；中英双语开箱即用，支持 RTL 布局。</p>
+        <p>实例级主题隔离、外部跟随同步、主题继承，内置 4 套预设；中英双语开箱即用，支持 RTL 布局。</p>
       </div>
       <div class="feature">
         <div class="feature-icon">