Extend custom events page with an explanation how to use event-impl-gen

[stephan-gh]

event-impl-gen can be used to automatically generate event implementations from event interfaces at compile time. That saves plugin developers from the boilerplate code to implement all the event properties, all they need to do is configure the Gradle plugin and use the generated event factory instead of custom implementation classes.

The Custom Events article should be extended with an explanation of how it can be used. The manual way (without class generation) should be still kept for those not using Gradle to build their plugin.

[ST-DDT]

@Minecrell Is this still a thing? If yes, could you please describe what needs to be done to actually use it? Or show me an plugin that uses it and can serve as an example?
(or write the docs yourself, if you want)