This commit reorganizes the asciidoctor attributes for the reference
documentation. Instead of being contributed partially by the build and
individual documents, attributes are now shared in common files that are
included in top sections.
This commit configures the documentation build to merge the entire
reference documentation into a single PDF file.
To achieve that, this commit also fixes duplicate anchors that were
introduced while factoring out common parts of the documentation for MVC
and WebFlux.
Closes gh-28451
