Consider moving javadoc generation outside of the normal build

[aaime]

We can speed the build this way, while maintaining control over javadoc generation by having a github action build them.

[jodygarnett]

I would love to build the javadocs and publish them on gh-pages, it would take some pressure off the origional oracle docs which act as an API reference ..

• https://eclipse-imagen.github.io/imagen/guide/api-summary/
• https://github.com/eclipse-imagen/imagen/wiki/Documentation-migration