Document some examples of Custom Login Modules using Elytron

We used to have an example of Custom Login Module in our EAP 6 document - https://access.redhat.com/documentation/en-US/JBoss_Enterprise_Application_Platform/6.3/html/Security_Guide/Custom_LoginModule_Example.html
In our EAP 7.1 document, we don't have any such example for doing the same in Elytron or legacy way - https://access.qa.redhat.com/documentation/en-us/red_hat_jboss_enterprise_application_platform/7.1.alpha/html-single/login_module_reference/#custom_login_modules

This needs to be added to our document.