Document AssertJ support for MockMvc

This commit restructures the section on MockMvc so that the anchors
are easier to read. The standard integration has moved to a
Hamcrest Integration section  at the same level as HtmlUnit Integration,
and a new AssertJ Integration section has been created.

Closes gh-32454

framework-docs/framework-docs.gradle

@@ -49,6 +49,7 @@ dependencies {
 	api(project(":spring-webmvc"))
 	api(project(":spring-context-support"))
 	api(project(":spring-aspects"))
+	api(project(":spring-test"))
 	api(project(":spring-websocket"))
 
 	api("org.jetbrains.kotlin:kotlin-stdlib")