Run `npm install --save grunt-cdn`, and then add these bits to Gruntfile.js

Gruntfile.js

grunt.initConfig({
  // ...
  cdn: {
    options: {
        /** @required - root URL of your CDN (may contains sub-paths as shown below) */
        cdn: 'https://YOUR-USERNAME.github.io/YOUR-REPO/',
        /** @optional  - if provided both absolute and relative paths will be converted */
        flatten: true
        /** @optional  - if provided will be added to the default supporting types */
        // supportedTypes: { 'phtml': 'html' }
    },
    dist: {
        /** @required  - string (or array of) including grunt glob variables */
        src: ['./dist/*.html', './dist/styles/{,*/}*{,*/}*.css']
        /** @optional  - if provided a copy will be stored without modifying original file */
        // dest: './dist/static/'
    }
  }
//...
})


grunt.registerTask('build', [
    //...
    'cdn:dist'
]);

grunt.loadNpmTasks('grunt-cdn');