Render reference documentation with Asciidoctor

This commit removes docbook from the documentation toolchain and
instead makes use of asciidoctor to render the reference documentation
in HTML and PDF formats.

The main Gradle build has been refactored with the documentation tasks
and sniffer tasks extracted to their own gradle file in the "gradle"
folder.

A new asciidoctor Spring theme is also used to render the HTML5 backend.

Issue: SPR-14997

src/docs/asciidoc/core.adoc

@@ -0,0 +1,44 @@
+[[spring-core]]
+= Core Technologies
+:doc-root: https://docs.spring.io
+:api-spring-framework: {doc-root}/spring-framework/docs/{spring-version}/javadoc-api/org/springframework
+
+[partintro]
+--
+This part of the reference documentation covers all of those technologies that are
+absolutely integral to the Spring Framework.
+
+Foremost amongst these is the Spring Framework's Inversion of Control (IoC) container. A
+thorough treatment of the Spring Framework's IoC container is closely followed by
+comprehensive coverage of Spring's Aspect-Oriented Programming (AOP) technologies. The
+Spring Framework has its own AOP framework, which is conceptually easy to understand,
+and which successfully addresses the 80% sweet spot of AOP requirements in Java
+enterprise programming.
+
+Coverage of Spring's integration with AspectJ (currently the richest - in terms of
+features - and certainly most mature AOP implementation in the Java enterprise space) is
+also provided.
+
+
+* <<beans>>
+* <<resources>>
+* <<validation>>
+* <<expressions>>
+* <<aop>>
+* <<aop-api>>
+--
+
+
+
+include::core/core-beans.adoc[leveloffset=+1]
+
+include::core/core-resources.adoc[leveloffset=+1]
+
+include::core/core-validation.adoc[leveloffset=+1]
+
+include::core/core-expressions.adoc[leveloffset=+1]
+
+include::core/core-aop.adoc[leveloffset=+1]
+
+include::core/core-aop-api.adoc[leveloffset=+1]
+