new tutorial on custom RHS functions and semidiscretizations

[ranocha]

This is a feature that has often been asked for on Slack/Discourse etc. Thus, I wrote a little tutorial explaining the concept of our RHS function and semidiscretization in Trixi.jl. In particular, it is now documented and becomes a part of the public API of Trixi.jl.

@j-salmeron Does this help you with your problem?

[github-actions]

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[sloede]

Nice work, and impressively detailed - I think this is going to be very helpful! Just a few suggestions, then it's good to go imho

[ranocha]

@sloede I also added the package versions used in the tutorial in 7747f99. I will create a new PR doing that for the other tutorials as well.

[sloede]

LGTM!

[ranocha]

LGTM!

Thanks!