Improved documentation

[cwittens]

This is the current version.
It can be found at: https://cwittens.github.io/DispersiveShallowWater.jl/dev/

It is still missing / has problems with:

• The chapter https://cwittens.github.io/DispersiveShallowWater.jl/dev/SBP_Operators/#3.-Types-of-SBP-Operators is missing. @JoshuaLampert can you maybe write it? I don't know too much about the various SBP operators, especially Fourier, vs DG vs CG
• In the Relaxation example, the error growth with and without relaxation is almost the same (maybe the problem is with the analytical solution? because the error is quite large).
• There is still extra code in make.jl and .github/workflows/Documenter.yml that is needed for compilation in my fork, but it needs to be deleted before merging.

[JoshuaLampert]

Just one small point: Could you add a NEWS entry?

[ranocha]

Thanks a lot! I am fine with this PR once the comments of @JoshuaLampert have been resolved. Feel free to merge when he approves this PR.