Redesign MockMvcHtmlUnitDriverBuilder API

This commit introduces a dedicated build() method in
MockMvcHtmlUnitDriverBuilder to replace createDriver(). In addition,
the configureDriver() method has been renamed to withDelegate() and now
returns the builder for further customization.

This commit also overhauls the Javadoc for static factory methods and
the class-level Javadoc in MockMvcHtmlUnitDriverBuilder for greater
clarity to end users.

Issues SPR-13158

src/asciidoc/testing.adoc

@@ -4594,7 +4594,7 @@ WebDriver driver;
 public void setup() {
 	driver = MockMvcHtmlUnitDriverBuilder
 		.webAppContextSetup(context)
-		.createDriver();
+		.build();
 }
 ----
 
@@ -4818,7 +4818,7 @@ following:
 def setup() {
 	browser.driver = MockMvcHtmlUnitDriverBuilder
 		.webAppContextSetup(context, springSecurity())
-		.createDriver()
+		.build()
 }
 ----