How can I configure zsh completion to show hidden files and folders?

This is a zsh option. It is mentioned in its introduction.

GLOBDOTS lets files beginning with a . be matched without explicitly specifying the dot.

Add the following to your .zshrc file.

setopt globdots

You could add globdots to $_comp_options in your .zshrc e.g.

.....
compinit
_comp_options+=(globdots)
.....