For compatibility with Gradle docbook-reference-plugin, which cannot
handle DocBook 4.

This change eliminates the spring-framework-reference subproject, moving
these sources into the root project's own src directory.

This makes sense because the reference docs span all submodules, and
also because api Javadoc is created at the root project level as well.
This means that both api and reference documentation output will now
reside in the root project's 'build' directory. This is more consistent
and easy to discover.

Issue: SPR-8591

All <section/> elements in beans.xml >=~ 500 lines have been broken out
into separate documents with DOCTYPE 'section'. This refactoring makes
working with these files much easier in wysiwyg editors (namely
oXygen Author).

For consistency, this same refactoring should be applied to all other
chapters much larger than 1500 lines, such as aop.xml (3861), mvc.xml
(3466), jdbc.xml (3042), and so on.

beans.xml and the new section files have also been formatted for
consistency and to avoid whitespace diffs as much as possible into the
future.