fix: use h2 for feature headers (#774)

Use the h1 element as a top-level heading only (all h1 elements are treated as top-level headings by many screen readers and other tools).

src/client/theme-default/components/VPBox.vue

@@ -9,7 +9,7 @@ defineProps<{
 <template>
   <article class="VPBox">
     <div v-if="icon" class="icon">{{ icon }}</div>
-    <h1 class="title">{{ title }}</h1>
+    <h2 class="title">{{ title }}</h2>
     <p class="details">{{ details }}</p>
   </article>
 </template>