Build an Appointment Scheduler using Node, Firebase, and Vonage

This tutorial guides through building an appointment scheduler web application using Node.js, Express, Firebase, and the Vonage Messages API. The project structure is set up with a `public` folder for static files, including HTML content and CSS styling, and a `server.js` file that handles Express requests and interacts with the Firebase Realtime Database and Vonage API. The code includes functions to check if a slot is available, add slots to the database, send SMS confirmations, and cancel appointments. The application uses Express for routing, Firebase Admin SDK for interacting with the database, and Vonage Messages API for sending SMS messages.