Add demo folder to repo

The README files mentions a demo folder with examples. This folder does not exist yet. But it would be an excellent place to start for new people, so we should definitely make it. What I am thinking about are the following examples:

• Load in origin SlicedShower, create TemplateSynthesis object, make a target Shower and synthesise (in Numpy)
• Adaptation of previous example in JAX
• Using the tools to create CORSIKA input files for sliced simulations
• Showing the tools for switching from GEO/CE to full 3D E-fields and transforming to other CS
• Adjusting the phases when changing the geometry