Add an extra hook point in `AbstractDispatcherServletInitializer` to
customize the `DispatcherServlet`.

Issue: SPR-13222

Issue: SPR-13120

A note is added to suggest the use of @Primary with a
LocalValidatorFactory bean next to the MVC Java config.

Issue: SPR-12194

Conversion and validation documentation related to Spring MVC is now
consolidated in the Spring MVC chapter with references to and from
the Validation and Data Binding chapter.

Examples have been updated to include MVC Java config as well.

This commit updates examples in the reference manual to use annotation
attribute aliases.

Issue: SPR-12323

Issue: SPR-13043

Issue: SPR-7773

Issue: SPR-12617

This commit adds support for script based templating. Any templating
library running on top of a JSR-223 ScriptEngine that implements
Invocable like Nashorn or JRuby could be used.

For example, in order to render Mustache templates thanks to the Nashorn
Javascript engine provided with Java 8+, you should declare the following
configuration:

@Configuration
@EnableWebMvc
public class MustacheConfig extends WebMvcConfigurerAdapter {

	@Override
	public void configureViewResolvers(ViewResolverRegistry registry) {
		registry.scriptTemplate();
	}

	@Bean
	public ScriptTemplateConfigurer configurer() {
		ScriptTemplateConfigurer configurer = new ScriptTemplateConfigurer();
		configurer.setEngineName("nashorn");
		configurer.setScripts("mustache.js");
		configurer.setRenderObject("Mustache");
		configurer.setRenderFunction("render");
		return configurer;
	}
}

The XML counterpart is:

<beans>
	<mvc:annotation-driven />

	<mvc:view-resolvers>
		<mvc:script-template />
	</mvc:view-resolvers>

	<mvc:script-template-configurer engine-name="nashorn" render-object="Mustache" render-function="render">
		<mvc:script location="mustache.js" />
	</mvc:script-template-configurer>
</beans>

Tested with:
 - Handlebars running on Nashorn
 - Mustache running on Nashorn
 - React running on Nashorn
 - EJS running on Nashorn
 - ERB running on JRuby
 - String templates running on Jython

Issue: SPR-12266

The use of RedirectAttributes was documented in four places in the
reference documentation. This commit merges some of the places and links
properly between them.

Issue: SPR-12759

Generally update chapter and add documentation for 4.2 including
the return value types ResponseBodyEmitter, SseEmitter, and
StreamingResponseBody.

Issue: SPR-12672

Issue: SPR-12831

Issue: SPR-11792

The upgrade to a more recent asciidoc format led to a bunch of of
broken Admonition as the text block now requires four equal signs while
previous versions tolerated only 3.

Issue: SPR-12791

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