Use Asciidoctor's revision line to provide document's version

Using the revision line means that the revision number is automatically
set. The revision number is then included by the HTML and PDF backends
in the standard location. In the HTML backend that's alongside the
list of authors. In the PDF backend that's on the title page.

Closes gh-26851

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/asciidoc/index.adoc

@@ -1,5 +1,7 @@
 = Spring Boot Gradle Plugin Reference Guide
 Andy Wilkinson, Scott Frederick
+v{gradle-project-version}
+:!version-label:
 :doctype: book
 :toc: left
 :toclevels: 4