SpringBoot Enable Links for the Javadoc of the Gradle Plugin

The main "api" Javadoc has its links configured here: https://github.com/spring-projects/spring-boot/blob/4c23d2c45b429f47633358ac77d9012d3815aa60/spring-boot-project/spring-boot-docs/build.gradle#L207-L214

The Javadoc for the Gradle Plugin does not have links at the moment.