docs: demo card (#6703)

src/lazyload/demo/index.vue

@@ -1,16 +1,20 @@
 <template>
   <demo-section>
     <demo-block :title="t('basicUsage')">
-      <img v-for="img in imageList" v-lazy="img" />
+      <img v-for="img in imageList" :key="img" v-lazy="img" />
     </demo-block>
 
     <demo-block :title="t('title2')">
-      <div v-for="img in backgroundImageList" v-lazy:background-image="img" />
+      <div
+        v-for="img in backgroundImageList"
+        :key="img"
+        v-lazy:background-image="img"
+      />
     </demo-block>
 
     <demo-block :title="t('title3')">
       <lazy-component>
-        <img v-for="img in componentImageList" v-lazy="img" />
+        <img v-for="img in componentImageList" :key="img" v-lazy="img" />
       </lazy-component>
     </demo-block>
   </demo-section>
@@ -68,7 +72,6 @@ export default {
     background-repeat: no-repeat;
     background-size: 100% 100%;
     border-radius: 12px;
-    box-shadow: 0 8px 12px #ebedf0;
   }
 
   .van-doc-demo-block__title,