Add PyTorch cheatsheet section to docs

[darsnack]

ParameterSchedulers.jl supports most (all?) of the schedules in PyTorch, but we don't use the same naming, and we push for composition over highly specific schedules. We should add a "PyTorch" cheatsheet section to the docs that is a how-to guide for recreating PyTorch schedulers in ParameterSchedulers. For example, we could show how ReduceLROnPlateau can be created with Stateful and Flux.plateau.