Remove unnecessary folders and files from PDF reference documentation

Prior to this commit, the asciidoctor Gradle task was configured to
generate both the HTML5 and PDF backends. Unfortunately, this resulted
in resources such as HTML, JavaScript, CSS, and images being published
alongside the generated PDF documents.

This commit addresses this issue by introducing the use of a dedicated
asciidoctorPdf Gradle task. The existing asciidoctor Gradle task has
been modified to only generate HTML5 output.

In addition, the asciidoctor task now has a dynamic dependency on the
asciidoctorPdf task if the current project version is a non-SNAPSHOT
version. Thus, invoking `./gradlew asciidoctor` will still generate both
the HTML5 and PDF outputs for non-SNAPSHOT versions; whereas,
`./gradlew asciidoctorPdf` will generate only the PDF outputs regardless
of the current project version.

See gh-25783

Author: sbrannen

gradle/docs.gradle

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  *  Licensed under the Apache License, Version 2.0 (the "License");
  *  you may not use this file except in compliance with the License.
@@ -13,6 +13,18 @@
  *  See the License for the specific language governing permissions and
  *  limitations under the License.
  */
+	
+import org.asciidoctor.gradle.AsciidoctorTask
+
+buildscript {
+	repositories {
+		maven { url "https://repo.spring.io/plugins-release" }
+	}
+	dependencies {
+		classpath("org.asciidoctor:asciidoctor-gradle-plugin:1.5.8")
+		classpath("org.asciidoctor:asciidoctorj-pdf:1.5.0-alpha.16")
+	}
+}
 
 task api(type: Javadoc) {
 	group = "Documentation"
@@ -80,7 +92,15 @@ dokka {
 	}
 }
 
+asciidoctorj {
+	version = '1.5.8'
+}
+
 asciidoctor {
+	// only output PDF documentation for non-SNAPSHOT builds
+	if (!project.getVersion().toString().contains("BUILD-SNAPSHOT")) {
+		dependsOn 'asciidoctorPdf'
+	}
 	sources {
 		include '*.adoc'
 	}
@@ -91,10 +111,6 @@ asciidoctor {
 	}
 	logDocuments = true
 	backends = ["html5"]
-	// only ouput PDF documentation for non-SNAPSHOT builds
-	if(!project.getVersion().toString().contains("BUILD-SNAPSHOT")) {
-		backends += "pdf"
-	}
 	options doctype: 'book', eruby: 'erubis'
 	attributes  'icons': 'font',
 			'idprefix': '',
@@ -107,7 +123,29 @@ asciidoctor {
 			stylesdir: 'stylesheets/',
 			stylesheet: 'main.css',
 			'spring-version': project.version
+}
 
+task asciidoctorPdf(type: AsciidoctorTask) {
+	sources {
+		include '*.adoc'
+	}
+	logDocuments = true
+	backends = ["pdf"]
+	options doctype: 'book', eruby: 'erubis'
+	attributes  'icons': 'font',
+			'idprefix': '',
+			'idseparator': '-',
+			docinfo: '',
+			revnumber: project.version,
+			sectanchors: '',
+			sectnums: '',
+			'source-highlighter': 'coderay@', // TODO switch to 'rouge' once supported by the html5 backend
+			stylesdir: 'stylesheets/',
+			stylesheet: 'main.css',
+			'spring-version': project.version
+	doLast {
+		project.delete("$asciidoctorPdf.outputDir/pdf/images")
+	}
 }
 
 task docsZip(type: Zip, dependsOn: ['api', 'asciidoctor', 'dokka']) {