Add an archetype for a gallery item with an example

This includes:

• the archetype itself
• A layout that has right/left arrows to move to next gallery items
• Add a telling example
• Evaluate how randomized order can be achieved in static sites (maybe: shuffle at generation time, we rebuild very often, so it feels random to the user, Christian:carousel from bootstrap)
• Write an email to all developers to ask for more gallery items and nag them to add items