Missing documentation of source_terms

[knstmrd]

The source_terms parameter of the SemidiscretizationHyperbolic is not really documented anywhere outside of the full Trixi.jl API page (briefly mentioned on the Behind the scenes of a 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).

[DanielDoehring]

This could probably done in conjunction with a page dedicated to how we measure convergence (method of manufactured solutions)

[ranocha]

For people finding this issue, you can have a look at https://trixi-framework.github.io/Trixi.jl/stable/tutorials/custom_semidiscretization/. But we should definitely document it better - maybe a new tutorial after (or before) "Adding a new scalar conservation law" and "Adding a non-conservative equation"?