Introducation for vNodes use-cases

In the `VNode & JSX` section, it introduced how to describe a vNode, but there's not a description how to use(render) it.

Here're some use cases:
```vue
<script setup>
const CustomButton = h('button')
</script>

<template>
  <CustomButton />

  <component :is="CustomButton" />
</template>
```