[General] Fold some code

[ocots]

@AnasXbouali Add details top fold. See https://github.com/control-toolbox/MagneticResonanceImaging.jl/blob/50a78b6095af374c355d2f8bd04fa30101f913ce/docs/src/saturation.md?plain=1#L182-L238.

[ocots]

Or https://documenter.juliadocs.org/stable/showcase/#Details-admonition.