Docs: Improve our site search

Our site search for docs.gitlab.com is better now, but we're not quite there yet. We need to improve it, to make it easier to identify which doc (link) has the answers we're looking for.

Suggestion:

Add a description to the doc frontmatter:

title: "Title of the page"
description: "blah blah"

and render it below the page title in the search window, like this:

@axil WDYT? Also, can you please give me admin access to Algolia?