Is there an ActivityPub-compliant way to get a list of communities on an instance?

I'm exploring the ActivityPub protocol, trying to get familiar with it. When I query the "lemmy.ml" root (as an example) the outbox is listed as "https://lemmy.ml//site_outbox", which doesn't exist. But querying "https://lemmy.ml/site_outbox", which does exist, just gets me an empty list.

I know there's a way to get a list of communities from "https://lemmy.ml/api/v3/community/list", but that's using Lemmy's own custom API, not ActivityPub.