Improved method to override Varnish

[Robadob]

Whilst I was unable to find such a guide in the docs (albeit it is described as possible in a couple of places), I just found it explained in this PR.

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]'

A current test lesson can be found at https://zkamvar.github.io/instructor-training/

If @zkamvar sees this, and wants me to put a PR into the docs just drop a reply to the issue!