The link was previously named "Compared to WebFlux", which is easy to
mix up with the various links to equivalent sections in the WebFlux
chapter. Here the links point to a small section comparing the Servlet
Async API to the WebFlux stack from a high perspective.
In this commit we eliminate most of these links, except at the
beginning of the Asynchronous section. We also add a small mention of
the Servlet configuration in the comparison paragraphs, since the
Configuring section is the one furthest from the comparison paragraphs
that used to have a link to it.
Closes gh-29694
a3a94950f6