Mention Groovy Markup Templates in ref doc

Add a section on Groovy Markup Template support and
reorder sections in the View Technologies chapter, to have in order:

* Thymeleaf
* Groovy Markup Template
* Velocity and Freemarker
* JSPs
* Script Templates
* other views...

Issue: SPR-12829

spring-webmvc/src/main/java/org/springframework/web/servlet/view/groovy/GroovyMarkupConfigurer.java

@@ -79,7 +79,7 @@ import org.springframework.util.StringUtils;
  * @author Rossen Stoyanchev
  * @since 4.1
  * @see GroovyMarkupView
- * @see <a href="http://beta.groovy-lang.org/docs/groovy-2.3.2/html/documentation/markup-template-engine.html">
+ * @see <a href="http://groovy-lang.org/templating.html#_the_markuptemplateengine">
  *     Groovy Markup Template engine documentation</a>
  */
 public class GroovyMarkupConfigurer extends TemplateConfiguration

spring-webmvc/src/main/java/org/springframework/web/servlet/view/groovy/GroovyMarkupView.java

@@ -45,7 +45,7 @@ import org.springframework.web.util.NestedServletException;
  * @since 4.1
  * @see GroovyMarkupViewResolver
  * @see GroovyMarkupConfigurer
- * @see <a href="http://beta.groovy-lang.org/docs/groovy-2.3.2/html/documentation/markup-template-engine.html">
+ * @see <a href="http://groovy-lang.org/templating.html#_the_markuptemplateengine">
  *     Groovy Markup Template engine documentation</a>
  */
 public class GroovyMarkupView extends AbstractTemplateView {