Switch to spring-asciidoctor-backends

This commit uses the new AsciiDoctor backend.

Closes gh-29162

gradle/docs.gradle

@@ -1,17 +1,14 @@
 configurations {
-	asciidoctorExt
+	asciidoctorExtensions
 }
 
 dependencies {
-	asciidoctorExt("io.spring.asciidoctor:spring-asciidoctor-extensions-block-switch:0.5.0")
+	asciidoctorExtensions "io.spring.asciidoctor.backends:spring-asciidoctor-backends:0.0.3"
 }
 
 repositories {
 	maven {
 		url "https://repo.spring.io/release"
-		mavenContent {
-			includeGroup "io.spring.asciidoctor"
-		}
 	}
 }
 
@@ -68,20 +65,6 @@ pluginManager.withPlugin("kotlin") {
 	}
 }
 
-task downloadResources(type: Download) {
-	def version = "0.2.5"
-	src "https://repo.spring.io/release/io/spring/docresources/" +
-			"spring-doc-resources/$version/spring-doc-resources-${version}.zip"
-	dest project.file("$buildDir/docs/spring-doc-resources.zip")
-	onlyIfModified true
-	useETag "all"
-}
-
-task extractDocResources(type: Copy, dependsOn: downloadResources) {
-	from project.zipTree(downloadResources.dest);
-	into "$buildDir/docs/spring-docs-resources/"
-}
-
 asciidoctorj {
 	def docRoot = 'https://docs.spring.io'
 	def docsSpringFramework = "${docRoot}/spring-framework/docs/${project.version}"
@@ -115,17 +98,19 @@ asciidoctorj {
  */
 asciidoctor {
 	baseDirFollowsSourceDir()
-	configurations 'asciidoctorExt'
+	configurations "asciidoctorExtensions"
 	sources {
 		include '*.adoc'
 	}
 	outputDir "$buildDir/docs/ref-docs/html5"
+	outputOptions {
+		backends "spring-html"
+	}
 	logDocuments = true
 	resources {
 		from(sourceDir) {
 			include 'images/*.png', 'css/**', 'js/**'
 		}
-		from extractDocResources
 	}
 }
 
@@ -135,7 +120,7 @@ asciidoctor {
  */
 asciidoctorPdf {
 	baseDirFollowsSourceDir()
-	configurations 'asciidoctorExt'
+	configurations 'asciidoctorExtensions'
 	sources {
 		include '*.adoc'
 	}