Add details on creating new `init` project templates

## Description

An initial overview of the changes for https://github.com/dita-ot/dita-ot/pull/4509 is provided in the [4.3 Release Notes](https://www.dita-ot.org/4.3/release-notes/#v4.3__init-preview).

This information should be extended with details on how to create new project templates, and moved to a permanent location in the topic hierarchy for future reference.
