Create repository template for downstream packages

With more and more interesting ideas and projects extending Trixi.jl in unconventional ways popping up, I think it would be nice to streamline the process of creating a new repository for an (experimental) downstream package.

My motivation is that sometimes there are interesting ideas that would include a lot of breaking changes or that are just too ugly to be easily merged, but would be still nice to keep around. Of course, it is always possible to do this in a branch or fork of Trixi.jl, but then it also means that there is near-zero visibility for the work (and even lower probability that someone else will pick it up at a later time).

With the recent/upcoming separation of [TrixiSmartShockFinder.jl](https://github.com/trixi-framework/TrixiSmartShockFinder.jl) and [TrixiShallowWater.jl](https://github.com/trixi-framework/TrixiShallowWater.jl), maybe we can make it so that creating a new downstream repo is as easy as creating a new repo from a [template repo](https://docs.github.com/en/repositories/creating-and-managing-repositories/creating-a-template-repository), throwing in all your ugly but goody implementations, and writing a few lines of documentation.

Start by reviewing the linked TrixiSmartShockFinder.jl and TrixiShallowWater.jl repositories alongside GitHub's template-repository documentation. Define what a downstream-package template should contain, how it should be created and maintained, and what documentation would let a new contributor use it. Done means the template structure and workflow are agreed and documented.

Written by the indexing model from the issue text.