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

Author: wilkinsona

spring-boot-project/spring-boot-actuator-autoconfigure/src/docs/asciidoc/index.adoc

@@ -1,5 +1,7 @@
 = Spring Boot Actuator Web API Documentation
 Andy Wilkinson
+v{gradle-project-version}
+:!version-label:
 :doctype: book
 :toc: left
 :toclevels: 4

spring-boot-project/spring-boot-docs/src/docs/asciidoc/attributes.adoc

@@ -1,3 +1,4 @@
+:!version-label:
 :doctype: book
 :idprefix:
 :idseparator: -

spring-boot-project/spring-boot-docs/src/docs/asciidoc/index.adoc

@@ -1,11 +1,10 @@
 [[spring-boot-reference-documentation]]
 = Spring Boot Reference Documentation
 include::authors.adoc[]
+v{spring-boot-version}
 :docinfo: shared
 include::attributes.adoc[]
 
-*{spring-boot-version}*
-
 The reference documentation consists of the following sections:
 
 [horizontal]

spring-boot-project/spring-boot-docs/src/docs/asciidoc/index.singleadoc

@@ -1,11 +1,10 @@
 [[spring-boot-reference-documentation]]
 = Spring Boot Reference Documentation
 include::authors.adoc[]
+v{spring-boot-version}
 :docinfo: shared
 include::attributes.adoc[]
 
-*{spring-boot-version}*
-
 include::legal.adoc[leveloffset=+1]
 include::documentation-overview.adoc[leveloffset=+1]
 include::getting-started.adoc[leveloffset=+1]

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

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

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