Add search functionality to docs website

[mauriceyap]

The Please website does not currently provide a way to search for content across the site. Using Google search is an option to achieve this, but it is inelegant and relies on its cache.

The idea is to implement content search on the client side, in keeping with the architecture of the existing simple static site.

[peterebden]

I'm not clear why we want to build this ourselves - it feels a lot lower effort to leverage Google, I wouldn't describe that as inelegant.
Maybe it's nice to have something on the site to search directly, but it's easy enough for that to just link back to Google with a site:please.build search