Switch Logstash VPR to Asciidoctor (#822)

Switches the core of the docs build for the Logstash Versioned Plugin
Reference from the unmaintained AsciiDoc project to the actively
developed Asciidoctor project.

Author: nik9000

conf.yaml

@@ -577,10 +577,12 @@ contents:
             current:    versioned_plugin_docs
             branches:   [ versioned_plugin_docs ]
             index:      docs/versioned-plugins/index.asciidoc
+            private:    1
             chunk:      1
             noindex:    1
             tags:       Logstash/Plugin Reference
             subject:    Logstash
+            asciidoctor: true
             sources:
               -
                 repo:   logstash-docs