Improve documentation

[ranocha]

• [ ] Introduction including some background material
• [x] Tutorials
  • [x] Standard ODE problems
  • [x] PDE problems with structured matrices
  • [x] Linear advection: dense, sparse
  • [x] Heat equation with Neumann BCs: dense, sparse, Tridiagonal
  • [ ] Maybe https://github.com/JoshuaLampert/2024_fokker_planck
• [ ] Benchmarks
• [ ] More/improved docstrings

Docstrings can be added/modified in the source code. Documenter.jl will recognize them automatically and add them to the API reference. New pages need to be created as Markdown files in docs/src and have to be added to the list at the end of docs/make.jl.

[ranocha]

I think there is more stuff we can/should add (see above)

[SKopecz]

Sorry, I only wanted to mention this in the PR. I was not aware that it would be closed once the PR is merged.