doc: use custom MathJax package and config to display \longrightleftharpoons correctly

[pjaap]

In a discussion with @Da-Be-Ru @jpthiele we developed a hack to display the broken math symbols in the current doc in https://j-fu.github.io/VoronoiFVM.jl/dev/plutostatichtml_examples/heterogeneous-catalysis/

Pluto.jl uses a different MathJax than Documenter.jl.
Especially, we need the LaTeX package mhchem.

The solution is not intended to be final. A good solution would require to extract the MathJax package from Pluto.jl and pass this to the MathJax3 call as a url keyword.

[Da-Be-Ru]

I believe there are several ways to extract the MathJax source url used by the Pluto.jl transitive dependency in the docs environment. My understanding is that the only thing determining this is the editor.html head in the Pluto.jl frontend (the not-the-src-yet property is later copied into a src property by the invoked setup_mathjax function after an initial setup is done).

A straightfoward way to extract this URL from the docs environment is to extract the local Pluto source directory from the dependency graph and regex it out of there.

But since this is not an issue specific to VoronoiFVM.jl, I would actually suggest to escalate this functionality up to ExampleJuggler.jl which is where the @docplutonotebooks macro resides after all. Maybe this macro should already provide the proper mathengine to the Documenter.HTML call as well.

I already hacked together an example solution that could later be incorporated into ExampleJuggler.jl.
If you add me as a collaborator to your fork, @pjaap, I can demonstrate it with a commit.

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[j-fu]

Checked locally - this works, great, thanks @pjaap .

@Da-Be-Ru : I think the PR is good enough in the moment.

ExampleJuggler has two ways of making Pluto notebooks accessible in the docs: rendered by Pluto itself in an iframe (using PlutoSliderServer.jl) or as plain HTML via PlutoStaticHTML.jl.
Extracting the mathjax version pluto to me appears to be a bit too brittle anyway.

[Da-Be-Ru]

ExampleJuggler has two ways of making Pluto notebooks accessible in the docs: rendered by Pluto itself in an iframe (using PlutoSliderServer.jl) or as plain HTML via PlutoStaticHTML.jl. Extracting the mathjax version pluto to me appears to be a bit too brittle anyway.

Maybe, but I think with hard-coded URLs, this current version could just lead to similar errors depending on whatever version of Pluto the environment will resolve to and where this particular version of Pluto will get its MathJax source files from.

I tried my hand at a rough demonstration of what I mean on this fork which seems to work for me with both PlutoSliderServer.jl and PlutoStaticHTML.jl.

[j-fu]

Ok, we should keep this as is now and wait for JuliaDocs/Documenter.jl#2549

[pjaap]

JuliaDocs/Documenter.jl#2549 looks like a better solution than what we did downstream here.
However, we still need to place the mhchem LaTeX package into the MathJax config. I wonder if there is a cleaner way than to overwrite the whole config.

[j-fu]

Just added this to the Documenter PR...

JuliaDocs/Documenter.jl@c17b762