References in docs can contain both URL and doi

[nikosavola]

In for example https://helgegehring.github.io/femwell/julia/waveguide_anisotropic.html, the reference shows both a URL and a doi link, which point to the same thing

I think it's fine for the bibtex file to contain the URLs, but would be nice if the bibtex processor could have logic to only show the doi if it exists and otherwise show the URL.

[HelgeGehring]

Currently, we're waiting for jupyter-book/mystmd#443 to be fixed, then we'll probably move the docs to mystmd which is way more actively maintained than jupyterbook.

So I'm totally in favor of fixing this, but I think a quick fix is just fine for this. And take fix it completely when we switch.

What about just just commenting out the URLs if there's is both for now?

[nikosavola]

Okay, can wait for migration in that case