This commit moves the asciidoctor *.adoc files to the default location
and removes the unnecessary asciidoctor plugin configuration as a
result.
This changes also reworks dependencies between tasks and adds the
missing publication configuration for generated artifacts. The entire
docs-specific script is moved from the main build.gradle file to a
"gradle/docs.gradle" script to improve readability of the main build.
Fixes gh-1797
10ae7796c6