Enable Links for the Javadoc of the Gradle Plugin

[larsgrefer]

The main "api" Javadoc has its links configured here:

spring-boot/spring-boot-project/spring-boot-docs/build.gradle

Lines 207 to 214 in 4c23d2c

	options.links = [
	"https://docs.oracle.com/javase/8/docs/api/",
	"https://docs.oracle.com/javaee/7/api/",
	"https://docs.spring.io/spring-framework/docs/${versionConstraints["org.springframework:spring-core"]}/javadoc-api/",
	"https://docs.spring.io/spring-security/site/docs/${versionConstraints["org.springframework.security:spring-security-core"]}/api/",
	"https://tomcat.apache.org/tomcat-${tomcatDocsVersion}-doc/api/",
	"https://www.thymeleaf.org/apidocs/thymeleaf/${versionConstraints["org.thymeleaf:thymeleaf"]}/"
	] as String[]

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