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/modules/ROOT/pages/testing/mockmvc/assertj.adoc

@@ -0,0 +1,16 @@
+[[mockmvc-tester]]
+= AssertJ Integration
+:page-section-summary-toc: 1
+
+The AssertJ integration builds on top of plain `MockMvc` with several differences:
+
+* There is no need to use static imports as both the requests and assertions can be
+crafted using a fluent API.
+* Unresolved exceptions are handled consistently so that your tests do not need to
+throw (or catch) `Exception`.
+* By default, the result to assert is complete whether the processing is asynchronous
+or not. In other words, there is no need for special handling for Async requests.
+
+`MockMvcTester` is the entry point for the AssertJ support. It allows to craft the
+request and return a result that is AssertJ compatible so that it can be wrapped in
+a standard `assertThat()` method.