Create Guide about using custom packaging

### Improvement proposal

On the [plugin developers index page](https://maven.apache.org/plugin-developers/index.html) there is a open TODO

> TODO: creating and using custom packaging (like [maven-archetype packaging](https://maven.apache.org/archetype/archetype-packaging/))


Start at the plugin developers index page and inspect the open TODO about creating and using custom packaging. Use the linked Maven Archetype packaging page as the available reference, then create a guide covering both creation and use of custom packaging and remove or replace the TODO when the guide is complete.

Written by the indexing model from the issue text.