Is there a specification or documention for Gradle plugin development?

Hi there,

I’m new to this forum but a long-time, grateful user of Gradle and a rather experienced Java developer.
Recently I’ve started to develop my first Gradle plugin (in Java) and was somewhat puzzled that I could not find any decent documentation about Gradle plugin development, especially if compared to the excellent user documentation available. But maybe I didn’t find the obvious so that’s why I’m asking:

Is there some kind of specification for Gradle plugins or is there more documentation besides chapters 39 and 40 of the user doc and the design-docs on Github?
Or is there some kind of reference plugin to use as an example? I mean something more complex than the greetings plugin, at least with IO and some (expensive) initialization code, think of JPA meta model code generation or similar complexity.

Thanks in advance!