Write a tutorial on uncertainties returned by our solvers

[nathanaelbosch]

Right now we have some plots with posterior uncertainties by choosing very large steps and low order solvers, but in most applications they will barely be visible (see e.g. #25). A dedicated tutorial about uncertainties could help clarify these issues.

Topics covered could include:

• Uncertainties in general: When are they large, when are they small? How can they be visualized best?
• EK0 vs EK1 uncertainties
• What is a "diffusion model" and how do they influence the resulting uncertainties

To get started actually writing the tutorial, and for syntax etc, have a look at the current tutorials

• https://github.com/nathanaelbosch/ProbNumDiffEq.jl/blob/main/docs/src/getting_started.md
• https://github.com/nathanaelbosch/ProbNumDiffEq.jl/blob/main/docs/src/dae.md