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. Both of these tasks now
share common configuration supplied by the updated asciidoctorj Gradle
task.

In addition, the asciidoctor task now depends on the asciidoctorPdf
task. Thus, invoking `./gradlew asciidoctor` will still generate both
the HTML5 and PDF outputs; whereas, `./gradlew asciidoctorPdf` will
generate only the PDF outputs. We may later decide to rework the tasks
to introduce a dedicated asciidoctorHtml task so that we can generate
the HTML outputs without having to generate the PDF outputs (which are
more time consuming).

See gh-25783

build.gradle

@@ -5,6 +5,7 @@ plugins {
 	id 'org.jetbrains.kotlin.jvm' version '1.3.72' apply false
 	id 'org.jetbrains.dokka' version '0.10.1' apply false
 	id 'org.asciidoctor.jvm.convert' version '2.4.0'
+	id 'org.asciidoctor.jvm.pdf' version '2.4.0'
 	id 'de.undercouch.download' version '4.1.1'
 	id "io.freefair.aspectj" version '4.1.6' apply false
 	id 'com.gradle.build-scan' version '3.2'

gradle/docs.gradle

@@ -130,29 +130,6 @@ asciidoctorj {
 		}
 	}
 	fatalWarnings ".*"
-}
-
-/**
- * Produce the Spring Framework Reference documentation
- * from "src/docs/asciidoc" into  "build/asciidoc/html5"
- */
-asciidoctor {
-	baseDirFollowsSourceDir()
-	configurations 'asciidoctorExt'
-	sources {
-		include '*.adoc'
-	}
-	outputDir "$buildDir/docs/ref-docs/"
-	resources {
-		from(sourceDir) {
-			include 'images/*', 'css/**', 'js/**'
-		}
-		from extractDocResources
-	}
-	logDocuments = true
-	outputOptions {
-		backends = ["html5", "pdf"]
-	}
 	options doctype: 'book', eruby: 'erubis'
 	attributes([
 			icons: 'font',
@@ -171,6 +148,41 @@ asciidoctor {
 	])
 }
 
+/**
+ * Generate the Spring Framework Reference documentation from "src/docs/asciidoc"
+ * in "build/docs/ref-docs/html5".
+ */
+asciidoctor {
+	dependsOn asciidoctorPdf
+	baseDirFollowsSourceDir()
+	configurations 'asciidoctorExt'
+	sources {
+		include '*.adoc'
+	}
+	outputDir "$buildDir/docs/ref-docs/html5"
+	logDocuments = true
+	resources {
+		from(sourceDir) {
+			include 'images/*', 'css/**', 'js/**'
+		}
+		from extractDocResources
+	}
+}
+
+/**
+ * Generate the Spring Framework Reference documentation from "src/docs/asciidoc"
+ * in "build/docs/ref-docs/pdf".
+ */
+asciidoctorPdf {
+	baseDirFollowsSourceDir()
+	configurations 'asciidoctorExt'
+	sources {
+		include '*.adoc'
+	}
+	outputDir "$buildDir/docs/ref-docs/pdf"
+	logDocuments = true
+}
+
 /**
  * Zip all docs (API and reference) into a single archive
  */