Add custom Pod template tutorial

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Adds a custom Pod template tutorial that teaches users how to:

• Create a Dockerfile that extends a Runpod base image.
• Configure container startup options (JupyterLab/SSH, application + services, or application only).
• Add Python dependencies and system packages.
• Pre-load machine learning models from Hugging Face, local files, or custom sources.
• Build and test your image, then push it to Docker Hub.
• Create a custom Pod template in the Runpod console
• Deploy a Pod using your custom template.