Docs: Add `Epic` support to the list of target types

What does this MR do and why?

Add Epic support to the list of target types in Webhook documentation

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

NA

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

#491777 (closed) #430080 (closed)