Add Grails 8 GORM Event Listeners guide

[sanjana2505006]

Summary

• Add guides/gorm-event-listeners/v8/ for writing and testing GORM domain event listeners (sync + async) on Grails 8.
• Register version 8 in conf/guides.yml, pointing at grails-guides/gorm-event-listeners branch grails8.
• Cover Data Services, @Subscriber audit logging, @Listener property updates, and unit/integration testing.

Test plan

• Companion PR for grails-guides/gorm-event-listeners (grails8) is open/merged
• Guide TOC and snippet includes render as expected after site build
• Clone steps in howto match the companion repo layout

[sanjana2505006]

@jamesfredley Please let me know if there's any scope for improvements