Add GitHub action for javadoc generation

[ljacqu]

Not sure if it's worth it but sometimes there's a hiccup in releasing a new version of ConfigMe to Nexus because the javadoc plugin is unhappy with a certain Javadoc. If we had a GitHub action that regularly ran it, it would be noticeable earlier rather than in the middle of the releasing process.

[ljacqu]

Relates to and basically duplicates #163 (although not completely apparent)