But why? If GitHub Pages is free, then why pay?
I'm sure there are more, but those are top of mind for me.
I'm assuming you already have a GitHub Pages site that uses Jekyll hosted on GitHub. If that is true, then the first thing that you'll want to do is grab these three files.
Thanks goes to Khalid Abuhakmeh for writing the scripts
Once you have these three files, ensure they are in the root of your public GitHub pages site (ex. something.github.io)
You'll want to go inside of your Azure Portal (or use the CLI tools) and create an App Service -> Web App. Once the site is deployed, then go to Deployment Options and select GitHub, your project and press OK.
You should see "Settup up Deployment Source..." in the notification windows. You'll probably want to wait a good 15 to 20 minutes for Azure to setup everything. You can stay on the Deployment Options blade and you should see the status of the deployment.
After a while you see a check mark that it completed successfully. Now you can navigate to the URL listed on the Overview blade.