feat: allow showing a dropdown on site title

[rigor789]

Feature request

What problem does this feature solve?

I was browsing the vue-router docs, and wanted to jump to the vuex docs. Currently there is no "easy" way to do this, and what I would consider a convenient solution is if the current site title was a dropdown with a list of related sites.

What does the proposed API look like?

// .vuepress/config.js
module.exports = {
  themeConfig: {
    sites: [
      { text: 'Vue Router', link: 'https://router.vuejs.org' },
      { text: 'Vuex', link: 'https://vuex.vuejs.org' },
    ]
  }
}

How should this be implemented in your opinion?

I believe we can easily implement this by reusing the existing DropdownLink component

And a new branch in the Navbar component

vuepress/lib/default-theme/Navbar.vue

Lines 15 to 20 in a7d9a1a

	<span
	ref="siteName"
	class="site-name"
	v-if="$siteTitle"
	:class="{ 'can-hide': $site.themeConfig.logo }"
	>{{ $siteTitle }}</span>

      <!-- sitesLinks would be a computed property so that it can automatically add $siteTitle as the root elements text -->
      <DropdownLink v-if="sites" :item="sitesLinks" />
      <span
        ref="siteName"
        class="site-name"
        v-else-if="$siteTitle"
        :class="{ 'can-hide': $site.themeConfig.logo }"
       >{{ $siteTitle }}</span>

Are you willing to work on this yourself?**

If this is something useful for anyone, sure!

[ulivz]

Thanks for your interest in VuePress, but it has been a long-term topic, See: #683 (comment)