Implement search page

[NoelDeMartin]

Added search page as discussed in #352.

Given that the Algolia credentials are not available yet, I've used the ones from v2 for development. If you want to do the same, add the following to src/.vuepress/config.js under themeConfig:

algolia: {
  appId: 'BH4D9OD16A',
  apiKey: '85cc3221c9f23bfbaa4e3913dd7625ea',
  indexName: 'vuejs',
  algoliaOptions: { facetFilters: ["version:2.6"] },
}

I've implemented this looking at the current implementation in v2 docs.

There are some improvements that can be done given the new architecture, for example falling back to VuePress-powered search if Algolia is not configured or unavailable. However, as we've discussed in #352, that would be a nice addition to VuePress itself. So I think it's best to have this for now that is very similar to what we had in v2 and work on the improvements in VuePress.

[NataliaTepluhina]

@NoelDeMartin thank you for working on this! I see we have this error on build: ReferenceError: location is not defined at VueComponent.data (src/.vuepress/components/search/index.vue:57:0). Could you please look into it and fix it?

[NoelDeMartin]

@NataliaTepluhina Thanks for the heads-up, I forgot about SSR 😅 It should be fixed now.

[NataliaTepluhina]

Thank you @NoelDeMartin, changes look good to me! I'm waiting for the answer from Algolia regarding new credentials (so far we're on the stage 'Are you able to modify this website?' and the silence after this but I hope they will come back soon)

As soon as we have them, I'll add Algolia to VuePress theme config 👍🏻