This commit replaces the Docbook+Asciidoctor documentation toolchain by
a single Asciidoctor generation process.

First, we need to unzip the contents of the Spring Asciidoctor
documentation resources provided by the
`io.spring.docsresources:spring-docs-resources` distribution zip. This
is done in a `/target/refdocs` folder. We then copy all files from
`src/main/asciidoc` to the same location, and then launch the generation
process.

Closes gh-12611