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

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

Issue: SPR-15190

Issue: SPR-14497

The CORS section is now ahead of
"Integrating with other web frameworks" and "WebSocket Support".

This commit also removes the corresponding deprecated Servlet MVC variant and updates DispatcherServlet.properties to point to RequestMappingHandlerMapping/Adapter by default.

Issue: SPR-14129

Closes gh-1073

See gh-1073

Core and Web chapters are important chapters in the Spring Framework
reference documentation, and splitting them in multiple files will
help to evolve the documentation while not creating too many files.

Issue: SPR-12309

The reference documentation is now organized in asciidoc bookparts.
Bookparts sections can be also put in separate files, when their
size and theme are important enough to justify that.

Documentation Layout:
```
index
  |-- overview
  |
  |-- whats-new
  |
  |-- core
  |   |-- core-beans
  |   |-- core-aop
  |-- testing
  |
  |-- data-access
  |
  |-- web
  |   |-- web-mvc
  |   |-- web-view
  |-- integration
  |
  |-- appendix
      |-- appx-spring-tld
      |-- appx-spring-form-tld
```

Supersedes and closes #641

Issue: SPR-12309