Prior to this commit, the reference documentation build with asciidoctor
would get the common "spring-docs-resources" as a dependency and then
use it when generating the docs.

As seen in #23124, this can cause problems since we'd like to
consistently resolve our dependencies. In this case, the
"spring-doc-resources" archive is not published on maven central since
it's not officially supported by the Spring team as an open source
project.

This commit updates the reference documentation build to get this
archive as a simple download task and avoid resolving it as a
dependency.

See gh-23282