We're seeing a convergence of document DBs adding relational features, and relational DBs adding document features. At this point I find the best of both worlds to simply be PG with JSONB.

    create table document (
      id uuid primary key default gen_random_uuid(),
      doc jsonb
    );

This alone will give you a huge number of the features that Mongo provides, but also the full power of Postgres for everything else.