Improve documentation

[ranocha]

• Introduction including some background material
• Tutorials
  • Standard ODE problems
  • PDE problems with structured matrices
    • Linear advection: dense, sparse
    • 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.