Add Javadoc links to commonly depended-on plugins

[daniel-beck]

https://github.com/jenkinsci/plugin-pom/blob/91ff1d280dc3a773334b5c62872b63e004e01918/pom.xml#L360-L362

It would be nice if commonly depended on plugins providing types exposed through dependent plugins (cloudbees-folder, branch-api, scm-api, git, etc.; mailer and matrix-auth probably not because of packages shared with core?) would have their Javadoc URLs added here so references between plugins' Javadoc sites work.

Also Stapler to have links to the various types (e.g. StaplerProxy).

Thoughts?

[jglick]

Alternately, publish an aggregated Javadoc URL.