fix(layout): fix sidebar trigger position and add collapsible demo

Sidebar was missing position:relative so the absolute-positioned
collapse trigger rendered relative to the wrong ancestor.

Author: wangdicoder

components/layout/demo/sidebar.md

@@ -0,0 +1,57 @@
+<demo>
+
+### Collapsible Sidebar
+
+Use `collapsible` to enable a built-in toggle trigger on the sidebar. Control the collapsed state with `collapsed` and `onCollapse`.
+
+```jsx live
+() => {
+  const { Sidebar, Header, Content } = Layout;
+  const [collapsed, setCollapsed] = React.useState(false);
+
+  const sidebarStyle = {
+    background: 'rgba(110, 65, 191, 0.84)',
+    color: '#fff',
+    textAlign: 'center',
+  };
+
+  const headerStyle = {
+    background: 'rgba(110, 65, 191, 0.7)',
+    color: '#fff',
+    height: '64px',
+    lineHeight: '64px',
+    textAlign: 'center',
+  };
+
+  const contentStyle = {
+    background: 'rgba(110, 65, 191, 0.98)',
+    color: '#fff',
+    minHeight: '200px',
+    lineHeight: '200px',
+    textAlign: 'center',
+  };
+
+  return (
+    <Layout>
+      <Sidebar
+        style={sidebarStyle}
+        collapsible
+        collapsed={collapsed}
+        onCollapse={setCollapsed}
+        width={200}
+        collapsedWidth={60}
+      >
+        <div style={{ padding: '16px 0' }}>
+          {collapsed ? '☰' : 'Sidebar'}
+        </div>
+      </Sidebar>
+      <Layout>
+        <Header style={headerStyle}>Header</Header>
+        <Content style={contentStyle}>Content</Content>
+      </Layout>
+    </Layout>
+  );
+}
+```
+
+</demo>

components/layout/index.md

@@ -1,4 +1,5 @@
 import Basic from './demo/basic.md'
+import Sidebar from './demo/sidebar.md'
 
 # Layout
 
@@ -23,6 +24,7 @@ const { Header, Footer, Content, Sidebar } = Layout;
 ## Examples
 
 <Basic />
+<Sidebar />
 
 
 ## API

components/layout/style/_index.scss

@@ -33,6 +33,7 @@
   box-sizing: border-box;
   display: flex;
   flex-direction: column;
+  position: relative;
   transition: all 200ms;
   background: var(--ty-layout-sidebar-bg);
   color: #fff;