Restore layout of docs zip with index.html at the top

Issue: SPR-16799

gradle/docs.gradle

@@ -123,10 +123,14 @@ task docsZip(type: Zip, dependsOn: ['api', 'asciidoctor', 'dokka']) {
 		into "javadoc-api"
 	}
 
-	from (asciidoctor.outputDir) {
+	from ("$asciidoctor.outputDir/html5") {
 		into "spring-framework-reference"
 	}
 
+	from ("$asciidoctor.outputDir/pdf") {
+		into "spring-framework-reference/pdf"
+	}
+
 	from (dokka) {
 		into "kdoc-api"
 	}