Move Spring Boot version to main index in reference guide

Closes gh-24848

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

@@ -2,6 +2,9 @@
 = Spring Boot Reference Documentation
 include::authors.adoc[]
 :docinfo: shared
+include::attributes.adoc[]
+
+*{spring-boot-version}*
 
 The reference documentation consists of the following sections:
 

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

@@ -4,6 +4,8 @@ include::authors.adoc[]
 :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-docs/src/docs/asciidoc/legal.adoc

 [legal]
 = Legal
 
-{spring-boot-version}
-
 Copyright © 2012-2021
 
 Copies of this document may be made for your own use and for distribution to