Background: To address https://issues.jboss.org/browse/ENTMQBR-1484 for Broker 7.2 release, we have added a new section "Modularizing Broker Configuration File". We have given a link to the upstream doc [1] for information about the sub-types XML file that can be added to the `broker.xml` file.

Document the sub-types[1] in the downstream guide.

[1] https://activemq.apache.org/artemis/docs/latest/configuration-index.html.