How to retrieve "user@server.tld" handle from actor's URL

@tcit and @Goffi it would be truly wonderful if you might spare a bit of time and update our crowdsourced “Implementers Guide” should it be incomplete on this subject :pray:

So that hopefully the next AP dev can find everything in one place :hugs:


Update: For now I just copied/pasted in the list of resources at the webfinger section of the guide.