Adds opensearch to the docs site

Closes https://gitlab.com/gitlab-com/gitlab-docs/issues/162

• Adds opensearch (search from the URL bar (tab search))
• Adds Schema.org microdata for Google search snippets
• https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/191: Integration with InstantSearch: https://community.algolia.com/instantsearch.js/v2/guides/opensearch.html

Tutorial: https://www.algolia.com/doc/tutorials/search-ui/ux-patterns/search-from-the-url-bar/

Review

From the Review App, the tab-search seems to work:

• Review Apps: http://docs-add-opensearch.178.62.207.141.xip.io/
• Review Apps (opensearch.xml): http://docs-add-opensearch.178.62.207.141.xip.io/opensearch.xml

But it returns no results:

I think we need a results page for this, like Algolia's InstantSearch page. @axil WDYT? [Or maybe we blocked site search for review apps?

References:

• https://www.algolia.com/doc/tutorials/search-ui/instant-search/build-an-instant-search-results-page/instantsearchjs/#before-starting
• https://community.algolia.com/instantsearch.js/v2/getting-started.html
• https://community.algolia.com/docsearch/documentation/