Docs: Make example code executable

[DanielDoehring]

See
#2146 (comment)
and
#2146 (comment)

which applies for instance to

Trixi.jl/docs/src/callbacks.md

Lines 91 to 101 in dcf1b58

	```julia
	julia> using Trixi, Plots
	julia> trixi_include(joinpath(examples_dir(), "tree_2d_dgsem", "elixir_acoustics_gaussian_source.jl"))
	julia> pd1 = PlotData1D(time_series, 1)
	julia> pd2 = PlotData1D(time_series, 2)
	julia> plot(pd1["p_prime"]); plot!(pd2["p_prime"], xguide="t")
	```

Originally posted by @DanielDoehring in #2146 (comment)