[Docs] Full text search for vue-router docs.

[nicedaichi3]

What problem does this feature solve?

Hello :)
I'd like to full text search for vue-router docs.
beforeRouteLeave and others do not hit in header search.

VuePress comes with built-in headers-based search - it automatically builds a simple search index from the title, h2 and h3 headers from all the pages.

Would you like to introduce Algolia like a Vuejs document ?

If you give Algolia's API key specific to this document, I'll fix the code and commit it.
https://vuepress.vuejs.org/theme/default-theme-config.html#algolia-search

You have something to worry about, please let me know.

Thank you.

[posva]

Oh yeah, that's something we could add

[nicedaichi3]

Hi @posva
Could you please give me a hint how to get Algolia API Key for this docs.
Probably I think I should process on this refs: vuejs/v2.vuejs.org#575