Add antora gradle plugin

You can now run the following to generate the antora site

./gradlew antora

It will appear at build/site/index.html

build.gradle

@@ -27,11 +27,23 @@ buildscript {
 	}
 }
 
+plugins {
+	id "io.github.rwinch.antora" version "0.0.2"
+}
+
+
 apply plugin: 'io.spring.convention.root'
 
 group = 'org.springframework.session'
 description = 'Spring Session'
 
+antora {
+    playbookFile = file("local-antora-playbook.yml")
+    // default no version (current version)
+    antoraVersion = "3.0.0-alpha.9"
+    arguments = ["--fetch"]
+}
+
 subprojects {
 	apply plugin: 'io.spring.javaformat'