Missing documentation of source_terms

The `source_terms` parameter of the `SemidiscretizationHyperbolic` is not really documented anywhere outside of the full [Trixi.jl API](https://trixi-framework.github.io/Trixi.jl/stable/reference-trixi/#Trixi.jl-API) page (briefly mentioned on the [Behind the scenes of a simulation setup](https://trixi-framework.github.io/Trixi.jl/stable/tutorials/behind_the_scenes_simulation_setup/#behind_the_scenes_simulation_setup) page), so how to add source terms to a simulation can be seen only from various examples relying on the method of manufactured solutions.
I feel like some explicit example of how source terms work would be helpful (and would also show up in various related searches, e.g. googling for "trixi.jl" "source term" mostly points to the full API reference page).