Fix docs build: use 'spring-asciidoctor-backends' extension to add UI theme (#808)

spring-cloud-app-broker-docs/build.gradle

@@ -22,7 +22,7 @@ plugins {
 description = "Spring Cloud App Broker Documentation"
 
 configurations {
-	docs
+	asciidoctorExtensions
 }
 
 dependencies {
@@ -35,7 +35,7 @@ dependencies {
 	implementation "io.projectreactor:reactor-core"
 	implementation "io.r2dbc:r2dbc-h2:1.0.0.RELEASE"
 
-	docs "io.spring.docresources:spring-doc-resources:0.2.5@zip"
+	asciidoctorExtensions "io.spring.asciidoctor.backends:spring-asciidoctor-backends:0.0.5"
 }
 
 javadoc {
@@ -43,19 +43,13 @@ javadoc {
 }
 
 task prepareAsciidocBuild(type: Sync) {
-	dependsOn configurations.docs
-	// copy doc resources
-	from {
-		configurations.docs.collect { zipTree(it) }
-	}
-	// and doc sources
+	// Copy doc sources
 	from "src/docs/asciidoc"
 	// to a temporary build directory
 	into "$buildDir/asciidoc"
 }
 
 asciidoctorPdf {
-	dependsOn prepareAsciidocBuild
 	baseDirFollowsSourceFile()
 
 	asciidoctorj {
@@ -78,6 +72,10 @@ asciidoctorPdf {
 asciidoctor {
 	dependsOn asciidoctorPdf
   	baseDirFollowsSourceFile()
+	configurations "asciidoctorExtensions"
+	outputOptions {
+		backends "spring-html"
+	}
 	sourceDir = file("$buildDir/asciidoc")
 	sources {
 		include '*.adoc'