Docs: use Makie.Figure() instead of Makie.plot() for an empty canvas #994

CTModels removes the zero-argument Makie.plot(; kwargs...) (type piracy: it redefined Makie.plot() for every session loading Makie and CTModels), see control-toolbox/CTModels.jl#466.

In the "Plot with Makie" section of docs/src/results/plot.md, replace

f = plot(; size=(800, 800))
plot!(f, sol)

by

f = Makie.Figure(; size=(800, 800))
plot!(f, sol)

This applies from the next CTModels release.