new tutorial: How to create and run a custom product image

[fhennig]

This tutorial should:

• explain where all the images are
• show how to use our images as base images
• give an example of where to deploy this (i.e. custom registry?)
• show how to use this custom image in a Stacklet definition
• show how to update a custom image

# Dockerfile
FROM docker.stackable.tech/stackable/spark-k8s:3.5.0-stackable23.11.0