Use discrete headings instead of titled blocks in reference manual

[sbrannen]

In various places in the reference manual, whenever we need a "section" that is not physically part of the table of contents, we currently use titled blocks in an attempt to create a pseudo-section with a title and an anchor for cross referencing that section.

For example:

.[[beans-java-combining-xml-centric-declare-as-bean]]Declaring `@Configuration` classes as plain Spring `<bean/>` elements
--
<content here>
--

Unfortunately, the title gets rendered in italics and directly underneath the preceding content as follows:

Whereas, we actually intend for that to be rendered like this:

To achieve the desired type setting, we should instead use Asciidoc's support for discrete headings, which allow the headings to be styled like section titles but without affecting the section hierarchy or table of contents.