feat(theme): add team page feature

[kiaking]

This PR adds a new "Team Page" feature. See more details in docs.

---
layout: page
---
<script setup>
import {
  VPTeamPage,
  VPTeamPageTitle,
  VPTeamMembers
} from 'vitepress/theme'

const members = [
  {
    avatar: 'https://www.github.com/yyx990803.png',
    name: 'Evan You',
    title: 'Creator',
    links: [
      { icon: 'github', link: 'https://github.com/yyx990803' },
      { icon: 'twitter', link: 'https://twitter.com/youyuxi' }
    ]
  },
  ...
]
</script>

<VPTeamPage>
  <VPTeamPageTitle>
    <template #title>Our Team</template>
    <template #lead>Lorem ipsum...</template>
  </VPTeamPageTitle>
  <VPTeamMembers :members="members" />
</VPTeamPage>

Why not frontmatter but component composition?

Because composing components are way more flexible.

Why not props for title and lead but instead slots?

Because it's way more flexible to add custom styles or even links inside a lead text.