Create docs about Newts confd parameters

It is not documented right now which Newts configurations can be done with confd in Docker based OpenNMS installations.

These configurations are available right now: https://github.com/OpenNMS/opennms/blob/develop/opennms-container/horizon/container-fs/confd/templates/newts.properties.tpl

For the Minion container we already have the options documented (at least in a markdown file) as you can see here: https://github.com/OpenNMS/opennms/blob/develop/opennms-container/minion/CONFD_README.md

We can do it the same for the Horizon container. The file
https://github.com/OpenNMS/opennms/tree/develop/opennms-container/horizon/CONFD_README.md

is not existing right now, but will exist when merging https://github.com/OpenNMS/opennms/pull/3205.