About custom domains and GitHub Pages→
GitHub Pages supports using custom domains, or changing the root of your site's URL from the default, like octocat.github.io, to any domain you own.
Managing a custom domain for your GitHub Pages site→
You can set up or update certain DNS records and your repository settings to point the default domain for your GitHub Pages site to a custom domain.
Troubleshooting custom domains and GitHub Pages→
You can check for common errors to resolve issues with custom domains or HTTPS for your GitHub Pages site.