Consider using MultiDocumenter.jl for the entire Trixi Framework and/or all

There's a new package [MultiDocumenter.jl](https://juliacomputing.github.io/MultiDocumenter.jl/docs/) that allows one to link to (and search!) Documenter-generated websites from a single interface. Since it also allows one to build a simple top navigation bar + add some original docs, maybe this could be interesting for us as a "main entry point" to the Trixi Framework ecosphere (instead of the [current one](https://trixi-framework.github.io/)), or at least as a common documentation home to the docs for all main Julia packages (Trixi.jl, TrixiAtmo.jl, TrixiSW.jl, TrixiP.jl, and possibly our common packages as well).

Then again, it might be too limited and/or documentation-focused for our purposes, but I'd be interested to hear your thoughts on this

Read the MultiDocumenter.jl documentation and compare its linked-site search, navigation, and original-docs features with the current Trixi Framework entry point. Review the documentation sites for Trixi.jl, TrixiAtmo.jl, TrixiSW.jl, TrixiP.jl, and common packages. Done means producing a concrete recommendation about whether and where to adopt it.

Written by the indexing model from the issue text.