Issue title: Prev / Next links
 Issue body: Ref: https://vuepress.vuejs.org/theme/default-theme-config.html#prev-next-links
 Comments: 
 Comment 0: This is a vital feature for me. Will be happy to help; try out; discuss approaches from a user point of view.
 Comment 1: Hi !
I tried to solve this issue, and created [PR](https://github.com/vuejs/vitepress/pull/56)!

I want vitepress core team to check my PR, but I'm not sure who to mention... 🤔 
@yyx990803  ?
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