Add banner slot

[brc-dd]

similar to @vue/theme (and adjust styles to respect --vp-banner-height)

[ylw5]

In @vue/theme, you should set style for banner like this
Banner.vue in vuejs/docs

<style>
html:not(.banner-dismissed) {
  --vt-banner-height: 60px;
}
</style>
<style scoped>
.banner {
  position: fixed;
  z-index: var(--vp-z-index-banner);
  box-sizing: border-box;
  top: 0;
  left: 0;
  right: 0;
  height: var(--vt-banner-height);
}

it's a little hard and inflexible for developer, if there is a better way to set banner that developer only focus banner's content

[kiaking]

Hmmm... How about we change VPNav to have position: sticky and then just insert banner on top of nav. And I think it would adjust the height automatically?

<div>
  <Banner />
  <div class="main">
    <VPNav />
  </div>
</div>

[kiaking]

No wait, forget about it, it wouldn't work... 😅