This commit removes the custom static resources for the reference
documentation and instead uses the new Spring Doc Resources project
https://github.com/spring-io/spring-doc-resources

The asciidoctor theme is now shared between Spring projects and
integrated in the project build.

This reverts commit dc253551.
The Reactor project reverted its Javadoc changes.

Reactor is now building its Javadoc with a recent JDK version, which
changes the structure of the published HTML pages.

Without this change, the build fails during the Dokka generation with
the following exception:

```
Exception while loading package-list from
ExternalDocumentationLinkImpl(
  url=http://projectreactor.io/docs/core/release/api/,
  packageListUrl=http://projectreactor.io/docs/core/release/api/package-list)
```

This commit now points to the `/element-list` page, which seems to
contain the same information as the `package-list` page.

This commit enables the `-use` javadoc flag so that class usage pages
are included in the aggregated JavaDoc that is published to
https://docs.spring.io/spring-framework/docs/.

Issue: SPR-17173

Issue: SPR-16799

Specify task "docsZip" source directory.
Issue: SPR-16789

This commit specifies a local packageListUrl and defines that dokka task
must be executed after the api task in order to be able to build KDoc
during the release process when the Spring Framework's Javadoc is not
published yet.

Issue: SPR-16687

Issue: SPR-16687

As of Gradle Kotlin plugin 1.1.60,
project.sourceSets.main.kotlin.srcDirs contains
Java source directories in addition to Kotlin
ones. This commit filter out Java sources to
retain only Kotlin ones.

Issue: SPR-16268

Properly separate tocbot from asciidoctor styles and use main.css to
include both.

Latest dependency updates (POI 3.17, Rome 1.8, EhCache 3.4, Caffeine 2.5.6, RxJava 2.1.4, Tomcat 8.5.21, JRuby 9.1.13, Rhino 1.7.7.2)

Issue: SPR-15659

This commit ensures that only the HTML version of the reference
documentation is rendered for SNAPSHOT builds. This speeds up
significatly the build.

Issue: SPR-15885

This commit reorders docs generation (javadoc, asciidoctor) *after*
runing tests for `./gradlew build` commands.

Issue: SPR-15885

Note: this syntax automatically applies the plugin to the root project.
Adding `apply false` in the plugin declaration disables that.

Issue: SPR-15885

This commit uses a tip provided by @mojavelinux to set the syntax
highlighter to Rouge for the PDF version of the User Guide.

Once Asciidoctor 1.5.7 has been released we will likely switch to Rouge
for the HTML version of the Reference Manual as well.

Issue: SPR-14997

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