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/web.adoc

@@ -0,0 +1,35 @@
+[[spring-web]]
+= The Web
+: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 Spring Framework's support for the
+presentation tier (and specifically web-based presentation tiers) including support
+for WebSocket-style messaging in web applications.
+
+Spring Framework's own web framework, <<mvc,Spring Web MVC>>, is covered in the
+first couple of chapters. Subsequent chapters are concerned with Spring Framework's
+integration with other web technologies, such as <<jsf,JSF>>.
+
+The section then concludes with comprehensive coverage of the Spring Framework
+<<websocket>> (including <<websocket-stomp>>).
+
+* <<mvc>>
+* <<view>>
+* <<web-integration>>
+* <<websocket>>
+--
+
+include::web/web-mvc.adoc[leveloffset=+1]
+
+include::web/web-view.adoc[leveloffset=+1]
+
+include::web/web-cors.adoc[leveloffset=+1]
+
+include::web/web-integration.adoc[leveloffset=+1]
+
+include::web/web-websocket.adoc[leveloffset=+1]
+
+include::web/web-flux.adoc[leveloffset=+1]