Tutorial ideas

This is an unfinished list of ideas for Trixi tutorials that might be interesting to have:

- [ ] A new tutorial on adding a new numerical flux
- [ ] A description of our `rhs!` as entry point for new developers
- [ ] Promote the tutorials even more!
- [ ] A tutorial on how to use the [`AnalysisCallback`](https://trixi-framework.github.io/Trixi.jl/stable/reference-trixi/#Trixi.AnalysisCallback) to extract analysis quantities over time, to add new error measures, and to add new integral analyses.
- [ ] A *video* tutorial (or possibly a series of shorter video tutorials) on the basics of Trixi.jl:
* How to get Julia and install Trixi.jl
* How to run your first elixir and do some small modifications to it
* Some explanations of the basic building blocks (equations, solver, mesh, callbacks, time integration)

Everyone please feel free to extend!

Start by reviewing the existing Trixi.jl tutorials, the `rhs!` entry point, and the `AnalysisCallback` reference linked in the issue. Clarify which tutorial idea to pursue and define its audience and scope; done means the selected tutorial is written, integrated with the existing tutorial material, and covers the agreed examples.

Written by the indexing model from the issue text.