This step is only applicable if you are using a custom domain! If you are using a <YOUR-USERNAME>.github.io domain, you can skip this step.
For this last bit to take effect, you also need to create a CNAME record with the DNS provider you register your domain with (i.e. NameCheap, Google Domains).
GitHub has some documentation on this, but the tldr; is to
github.com/<YOUR-GITHUB-USERNAME>/quartz) settings page and go to the Pages tab. Under "Custom domain", type your custom domain, then click Save.Go to your DNS Provider and create a CNAME record that points from your domain to <YOUR-GITHUB-USERNAME.github.io. (yes, with the trailing period).
*Example Configuration for Quartz*