Add tutorial for building a dual-mode Serverless worker

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This PR adds a new tutorial document on building a dual-mode Serverless worker, promoting a "Pod-first" development workflow.

This guide is designed to help users overcome common challenges in Serverless development, such as slow iteration times and debugging difficulties. It introduces a method for creating a flexible Docker image that can operate as both a RunPod Pod for interactive development and a Serverless worker for production.

The tutorial provides step-by-step instructions for:

• Setting up a project with handler.py, start.sh, and Dockerfile.
• Creating a handler that adapts its behavior based on a MODE_TO_RUN environment variable.
• Writing a startup script to manage different operational modes.
• Building and deploying the Docker image.
• Testing the worker in both Pod and Serverless environments.