FR: Find and backlink site.standard.document for given URL #32

I am thinking of more and more ways that we can integrate with standard.site in general, not just with Margin. One of the things that I think would be really super cool is if when someone made a Margin annotation on a given URL, we did some work trying to see if there are any documents with the given URL and then if so we could put a backlink in the annotation, or the highlight, or the bookmark, or whatever, that would actually link that in the PDS. I should do some research and see if anyone is running an indexer on standard.site to get just like a bulk list of documents with their URLs we could match against.