add search to header

[swalx]

I've been using UIkit for some time now and although I know most of the components I still find myself using the docs each time I start a project. Since I have been using this framework, I've thought that a search feature would be really useful rather than scrolling through the side nav.

This contribution is not perfect and has some bugs but it's a step forward.

[janschoenherr]

Thanks, we are using Algolia now.