Adds arxiv plugin to the plugins directory in docs

[agustif]

I made https://github.com/agustif/llm-arxiv to make it easy to load arxiv papers as fragments,it takes either a full url or just arXiv id like -f arxiv:2504.17544v1

[simonw]

This plugin is really cool - I filed an issue here which I'd like to fix before adding it to the directory:

• Storing images as fragments doesn't quite work agustif/llm-arxiv#2

[agustif]

fixed conflicts would love to get this merged @simonw