Add a search bar to header section

[s-gbz]

Hi there,
I come back to your testing series quite frequently.
Today I noticed that I removed my bookmark, because I thought I was done with it (seems I wasn't).

It led me to notice that there's no option to search your site for specific content. Here's my proposal:

• Looks clean
• Easy to integrate
• Additional user comfort

Check here for a live example using lua search engine.
I might as well create the PR if you'd like.

Cheers

PS: While preparing this issue I noticed that the header section disappears altogether on mobile view.
Is that intentional? Might be one issue more ;) :)

[s-gbz]

Actually there might be way more viable alternatives, e.g. https://github.com/nextapps-de/flexsearch <- supposed to be quite good as well.