Improve documentation

[JoshuaLampert]

Up to now we only have one overview page in the documentation, which tries to cover everything in the package. I think it would be nice to split this up in several parts focusing on different aspects (e.g. pages about more theoretical aspects and more tutorial-like pages). Ideas for tutorials:

• Basic introductory example (also explain about SBP operators)
• Compare different equations applied to the Dingemans setup
• Compare different SBP solvers: Especially upwind vs central (upwind leads to higher resolution), but also FD vs. DG vs. CG vs. Fourier: Maybe make this two separate tutorials?
• Explain influence of using relaxation (exact preservation of nonlinear invariant, quadratic vs. linear error growth)

In addition, it would be nice to have a page explaining the implementation strategy and the interplay between different building blocks (mesh, equations, solver, semidiscretization, ODE solver, callbacks), maybe similar to the diagram in https://trixi-framework.github.io/Trixi.jl/stable/overview/#overview-semidiscretizations.

[JoshuaLampert]

If you have any other ideas, feel free to add or change the above comment.

[ranocha]

Sounds nice! We should also point out the general interface idea (abstract SWEs etc.).