Improve the Custom Modules section on the EAP 7.x OpenShift documentation

The 7.13.1.1. Custom Modules section on the Getting Started with JBoss EAP for OpenShift Container Platform describes how to create a custom module when you are using the EAP Image on OpenShift.

The description of this section is quite poor and also it points out an external example that is out of the control of the EAP team. This external project doesn't work at all with EAP 7.4 since it uses an obsolete server configuration file, in addition, this section doesn't explain well how you can run this specific example, the example application itself is not well documented either.

Since creating a custom module is a very common use case, we should improve the documentation of this section by adding a better description of how to build a custom module and removing the link that points out to an external application.

It doesn't need to be a practical example, just the steps, and the options you have to create a custom module.

This KBase will help in order to describe the available options: KCS: Creating a custom module in EAP 7 in OCP image