Publish Maven plugin API

This commit enables javadoc generator for the Spring Boot Maven plugin.

This also harmonizes the structure of the documentation, with an `/api`
and a `/reference` root directories for the javadoc and the reference
guide respectively.

Closes gh-20127

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/build.gradle

@@ -68,6 +68,19 @@ asciidoctorPdf {
 	}
 }
 
+javadoc {
+	options {
+		author = true
+		docTitle = "Spring Boot Maven Plugin ${project.version} API"
+		encoding = "UTF-8"
+		memberLevel = "protected"
+		outputLevel = "quiet"
+		splitIndex = true
+		use = true
+		windowTitle = "Spring Boot Maven Plugin ${project.version} API"
+	}
+}
+
 tasks.withType(org.asciidoctor.gradle.jvm.AbstractAsciidoctorTask) {
 	attributes	"maven-jar-plugin-version": "1.2.3",
 				"maven-failsafe-plugin-version": "1.2.3",
@@ -78,10 +91,13 @@ task zip(type: Zip) {
 	dependsOn asciidoctor, asciidoctorPdf
 	duplicatesStrategy "fail"
 	from(asciidoctorPdf.outputDir) {
-		into "pdf"
+		into "reference/pdf"
 	}
 	from(asciidoctor.outputDir) {
-		into "html"
+		into "reference/html"
+	}
+	from(javadoc) {
+		into "api"
 	}
 }