Add documentation about connecting DoltLab to external PostgreSQL server

[coffeegoddd]

In the docker-compose.yaml of DoltLab, doltlabapi can accept two arguments for connecting to an external postgres server, -pghost and -pgport.

During initialization, run:

CREATE ROLE dolthubadmin WITH LOGIN PASSWORD '$DOLTHUBAPI_PASSWORD';
CREATE DATABASE dolthubapi;
GRANT ALL PRIVILEGES ON DATABASE dolthubapi TO dolthubadmin;
CREATE EXTENSION citext SCHEMA public;
GRANT ALL PRIVILEGES ON ALL TABLES IN SCHEMA public TO dolthubadmin;
ALTER DEFAULT PRIVILEGES IN SCHEMA public GRANT ALL ON TABLES TO dolthubadmin;

This needs to be in Administration guide dolthub/docs#753