Enable search

[zkamvar]

This is paired with carpentries/varnish#86 and is most certainly not ready.

To configure a lesson to test this (do not put this in production), fork it, and then add these lines to your config.yaml:

sandpaper: carpentries/sandpaper@enable-search
varnish: carpentries/varnish@use-search-2023
url: '[user].github.io/[repo]'