Publish documentation under reference/html folder

This is done:
 - For consistency with other Spring projects
 - Because it is probably a better option to use reference instead of
   spring-framework-reference (spring-framework is already present in
   the URL)
 - To improve PDF documentation discoverability

As a consequence:
 - spring-framework-reference HTML documentation is now published
   under reference/html
 - spring-framework-reference/pdf PDF documentation is now published
   under reference/pdf

The following permanent redirect should be created before Spring
Framework 5.3.0 release:
 - current/spring-framework-reference/pdf/* to current/reference/pdf/*
 - current/spring-framework-reference/* to current/reference/html/*

Closes gh-25071

gradle/docs.gradle

@@ -188,10 +188,10 @@ task docsZip(type: Zip, dependsOn: ['api', 'asciidoctor', 'dokka']) {
 		into "javadoc-api"
 	}
 	from ("$asciidoctor.outputDir/html5") {
-		into "spring-framework-reference"
+		into "reference/html"
 	}
 	from ("$asciidoctor.outputDir/pdf") {
-		into "spring-framework-reference/pdf"
+		into "reference/pdf"
 	}
 	from (dokka) {
 		into "kdoc-api"