Define the reference doc authors in a single place

Closes gh-20896

spring-boot-project/spring-boot-docs/src/main/asciidoc/authors.adoc

+Phillip Webb; Dave Syer; Josh Long; Stéphane Nicoll; Rob Winch; Andy Wilkinson; Marcel Overdijk; Christian Dupuis; Sébastien Deleuze; Michael Simons; Vedran Pavić; Jay Bryant; Madhura Bhave; Eddú Meléndez

spring-boot-project/spring-boot-docs/src/main/asciidoc/index.htmladoc

 [[spring-boot-reference-documentation]]
 = Spring Boot Reference Documentation
-Phillip Webb, Dave Syer, Josh Long, Stéphane Nicoll, Rob Winch, Andy Wilkinson, Marcel Overdijk, Christian Dupuis, Sébastien Deleuze, Michael Simons, Vedran Pavić, Jay Bryant, Madhura Bhave, Eddú Meléndez
+include::authors.adoc[]
 :docinfo: shared
 
 The reference documentation consists of the following sections:

spring-boot-project/spring-boot-docs/src/main/asciidoc/index.htmlsingleadoc

 [[spring-boot-reference-documentation]]
 = Spring Boot Reference Documentation
-Phillip Webb, Dave Syer, Josh Long, Stéphane Nicoll, Rob Winch, Andy Wilkinson, Marcel Overdijk, Christian Dupuis, Sébastien Deleuze, Michael Simons, Vedran Pavić, Jay Bryant, Madhura Bhave
+include::authors.adoc[]
 :docinfo: shared
 include::attributes.adoc[]
 

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-reference.pdfadoc

 [[spring-boot-reference-documentation]]
 = Spring Boot Reference Documentation
-Phillip Webb, Dave Syer, Josh Long, Stéphane Nicoll, Rob Winch, Andy Wilkinson, Marcel Overdijk, Christian Dupuis, Sébastien Deleuze, Michael Simons, Vedran Pavić, Jay Bryant, Madhura Bhave
+include::authors.adoc[]
 :docinfo: shared
 include::attributes.adoc[]