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/htmlunit.adoc

@@ -0,0 +1,21 @@
+[[mockmvc-server-htmlunit]]
+= HtmlUnit Integration
+:page-section-summary-toc: 1
+
+Spring provides integration between xref:testing/mockmvc/overview.adoc[MockMvc] and
+https://htmlunit.sourceforge.io/[HtmlUnit]. This simplifies performing end-to-end testing
+when using HTML-based views. This integration lets you:
+
+* Easily test HTML pages by using tools such as
+  https://htmlunit.sourceforge.io/[HtmlUnit],
+  https://www.seleniumhq.org[WebDriver], and
+  https://www.gebish.org/manual/current/#spock-junit-testng[Geb] without the need to
+  deploy to a Servlet container.
+* Test JavaScript within pages.
+* Optionally, test using mock services to speed up testing.
+* Share logic between in-container end-to-end tests and out-of-container integration tests.
+
+NOTE: MockMvc works with templating technologies that do not rely on a Servlet Container
+(for example, Thymeleaf, FreeMarker, and others), but it does not work with JSPs, since
+they rely on the Servlet container.
+