The link would be 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, as Configuring section is
the one furthest from the comparison paragraphs that used to have a link
to it.

The block title style previously used was not rendered in HTML and the
title wouldn't be differentiated from the text. It was in the PDF, as
italics.

Introducing delimited blocks in the open (`--`) style did introduce
styling, but the vertical alignment isn't great.

This commit turns these block titles to actual (deep) section titles.
In the final HTML, at this depth there is no numbering but bold styling
is there. The PDF rendering has also be verified to have relevant style.