Implement Algolia search for about.gitlab.com

During the summit Sid and I talked about this and he asked me to follow up with @aherrmann and ask him to try to implement it asap.

We implemented Algolia for the GitLab Docs and after a few iterations and adjustments on how to index the site and display the results, it's working pretty well; we can actually find pretty much anything there.

It's particularly important for the handbook and the blog. Beyond that, if we make a separate index for the blog, we can consider adding it to the docs search page (https://docs.gitlab.com/search/) as an option to include blog tutorials in the docs search results. By doing so, we can help our users to find the tutorials burried in the blog backlog and make them more useful/discoverable on the long term.

Ashton can you please look into it? Pls lemme know if you need help.

cc/ @gl-docsteam @rebecca