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
This commit creates a new "framework-docs" module, which is dedicated to
documentation generation (reference and API docs).
This commit refactors the build configuration and moves the asciidoc
files to a separate location, but does not change the name nor the
nature of published artifacts.
Closes gh-29417
