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You would enqueue it, this is how Javascript is added to WordPress properly.

function my_scripts_intro() {
     is_front_page(){
         wp_enqueue_script( 'introjs','path to ..intro.js' );
         wp_enqueue_style( 'introcss','path to ..intro.css' );
        }
}
    
add_action( 'wp_enqueue_scripts', 'my_scripts_intro' );

To determine the state of the visitor you would probably want to use cookies.