Download docs resources as zip file in asciidoc build

Prior to this commit, the reference documentation build with asciidoctor
would get the common "spring-docs-resources" as a dependency and then
use it when generating the docs.

As seen in #23124, this can cause problems since we'd like to
consistently resolve our dependencies. In this case, the
"spring-doc-resources" archive is not published on maven central since
it's not officially supported by the Spring team as an open source
project.

This commit updates the reference documentation build to get this
archive as a simple download task and avoid resolving it as a
dependency.

See gh-23282

build.gradle

 buildscript {
 	repositories {
-		maven { url "https://repo.spring.io/plugins-release" }
+		maven { url 'https://repo.spring.io/plugins-release' }
 	}
 	dependencies {
-		classpath("io.spring.nohttp:nohttp-gradle:0.0.3.RELEASE")
-		classpath("org.asciidoctor:asciidoctorj-pdf:1.5.0-alpha.16")
-		classpath("io.spring.asciidoctor:spring-asciidoctor-extensions:0.1.3.RELEASE")
+		classpath 'org.asciidoctor:asciidoctorj-pdf:1.5.0-alpha.16'
+		classpath 'io.spring.asciidoctor:spring-asciidoctor-extensions:0.1.3.RELEASE'
 	}
 }
-
-// 3rd party plugin repositories can be configured in settings.gradle
 plugins {
-	id 'org.springframework.build.test-sources' apply false
-	id "io.spring.dependency-management" version "1.0.7.RELEASE" apply false
-	id "org.jetbrains.kotlin.jvm" version "1.3.41" apply false
-	id "org.jetbrains.dokka" version "0.9.18" apply false
-	id "org.asciidoctor.convert" version "1.5.8"
+	id 'io.spring.dependency-management' version '1.0.7.RELEASE' apply false
+	id 'org.jetbrains.kotlin.jvm' version '1.3.41' apply false
+	id 'org.jetbrains.dokka' version '0.9.18' apply false
+	id 'org.asciidoctor.convert' version '1.5.8'
+	id 'io.spring.nohttp' version '0.0.3.RELEASE'
+	id 'de.undercouch.download' version '4.0.0'
 }
 
 ext {

gradle/docs.gradle

@@ -79,22 +79,16 @@ dokka {
 	}
 }
 
-configurations {
-	docs
+task downloadResources(type: Download) {
+	def version = "0.1.2.RELEASE"
+	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")
 }
 
-dependencies {
-	// for documentation resources
-	docs "io.spring.docresources:spring-doc-resources:0.1.0.RELEASE@zip"
-}
-
-task extractDocResources(type: Sync) {
-	dependsOn configurations.docs
-	from {
-		configurations.docs.collect { zipTree(it) }
-	}
-	from "src/docs/asciidoc/"
-	into "$buildDir/asciidoc/build"
+task extractDocResources(type: Copy, dependsOn: downloadResources) {
+	from project.zipTree(downloadResources.dest);
+	into "$buildDir/docs/spring-docs-resources/"
 }
 
 /**
@@ -102,14 +96,15 @@ task extractDocResources(type: Sync) {
  * from "src/docs/asciidoc" into  "build/asciidoc/html5"
  */
 asciidoctor {
-	sourceDir "$buildDir/asciidoc/build"
 	sources {
 		include '*.adoc'
 	}
+	outputDir "$buildDir/docs/ref-docs/"
 	resources {
 		from(sourceDir) {
 			include 'images/*', 'css/**', 'js/**'
 		}
+		from "$buildDir/docs/spring-docs-resources/"
 	}
 	logDocuments = true
 	backends = ["html5"]
@@ -121,10 +116,10 @@ asciidoctor {
 	attributes  'icons': 'font',
 			'idprefix': '',
 			'idseparator': '-',
-			docinfo: 'shared',
-			revnumber: project.version,
-			sectanchors: '',
-			sectnums: '',
+			'docinfo': 'shared',
+			'revnumber': project.version,
+			'sectanchors': '',
+			'sectnums': '',
 			'source-highlighter=highlight.js',
 			'highlightjsdir=js/highlight',
 			'highlightjs-theme=atom-one-dark-reasonable',