vitepress 增加返回顶部功能

vitepress 返回顶部按钮，通过插槽的方式调整layout， 如何实现

具体组件内容不展示了，现在AI能轻松实现

这里主要说，vitepress 的插槽功能，这样通过插槽可以不改变原layout，最大程度的减少变动。

可以用的插槽还有很多哦

好喜欢这种预留的设计！！

<template>
  <div v-if="frontmatter.layout !== false" class="Layout" :class="frontmatter.pageClass" >
    <slot name="layout-top" />
    <VPSkipLink />
    <VPBackdrop class="backdrop" :show="isSidebarOpen" @click="closeSidebar" />
    <VPNav>
      <template #nav-bar-title-before><slot name="nav-bar-title-before" /></template>
      <template #nav-bar-title-after><slot name="nav-bar-title-after" /></template>
      <template #nav-bar-content-before><slot name="nav-bar-content-before" /></template>
      <template #nav-bar-content-after><slot name="nav-bar-content-after" /></template>
      <template #nav-screen-content-before><slot name="nav-screen-content-before" /></template>
      <template #nav-screen-content-after><slot name="nav-screen-content-after" /></template>
    </VPNav>
    <VPLocalNav :open="isSidebarOpen" @open-menu="openSidebar" />

    <VPSidebar :open="isSidebarOpen">
      <template #sidebar-nav-before><slot name="sidebar-nav-before" /></template>
      <template #sidebar-nav-after><slot name="sidebar-nav-after" /></template>
    </VPSidebar>

    <VPContent>
      <template #page-top><slot name="page-top" /></template>
      <template #page-bottom><slot name="page-bottom" /></template>

      <template #not-found><slot name="not-found" /></template>
      <template #home-hero-before><slot name="home-hero-before" /></template>
      <template #home-hero-info-before><slot name="home-hero-info-before" /></template>
      <template #home-hero-info><slot name="home-hero-info" /></template>
      <template #home-hero-info-after><slot name="home-hero-info-after" /></template>
      <template #home-hero-actions-after><slot name="home-hero-actions-after" /></template>
      <template #home-hero-image><slot name="home-hero-image" /></template>
      <template #home-hero-after><slot name="home-hero-after" /></template>
      <template #home-features-before><slot name="home-features-before" /></template>
      <template #home-features-after><slot name="home-features-after" /></template>

      <template #doc-footer-before><slot name="doc-footer-before" /></template>
      <template #doc-before><slot name="doc-before" /></template>
      <template #doc-after><slot name="doc-after" /></template>
      <template #doc-top><slot name="doc-top" /></template>
      <template #doc-bottom><slot name="doc-bottom" /></template>

      <template #aside-top><slot name="aside-top" /></template>
      <template #aside-bottom><slot name="aside-bottom" /></template>
      <template #aside-outline-before><slot name="aside-outline-before" /></template>
      <template #aside-outline-after><slot name="aside-outline-after" /></template>
      <template #aside-ads-before><slot name="aside-ads-before" /></template>
      <template #aside-ads-after><slot name="aside-ads-after" /></template>
    </VPContent>

    <VPFooter />
    <slot name="layout-bottom" />
  </div>
  <Content v-else />
</template>