Add Javadoc links to commonly depended-on plugins

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?