Document AssertJ support for MockMvc

[snicoll]

With #21178 merged, we still need to work on the documentation. The current MockMvc section is really focused on Hamcrest matchers and this makes some section harder to document that I would like. We've discussed to perhaps use different tabs for code sample to showcase "plain" MockMvc vs. the AssertJ counterpart but, with Kotlin, this leads to 4 tabs which is far from ideal.

[bclozel]

Should we consider separate pages for MockMvc/MockMvcTester and treat them as variants, just the way we do it for Servlet and Reactive? Each section could point to the relevant one like this one and this one.