Guide: How to configure OIDC manually via the Elytron subsystem for more advanced control of the configuration

[fjuma]

For more advanced use cases, one might want to configure OIDC manually via the Elytron subsystem for more control over the configuration (e.g., advanced role configuration, aggregate realms, etc.). The steps for manual configuration are described here:

https://issues.redhat.com/browse/WFLY-18829?focusedId=24583623&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-24583623

We should create a new guide that explains this.