Add docs in docs/index.asciidoc

[jordansissel]

The current documentation exists embedded as comments in the plugin. This is no longer used for plugin docs. Instead, we recently moved to putting the documentation in docs/index.asciidoc

Example: https://github.com/logstash-plugins/logstash-output-elasticsearch/blob/master/docs/index.asciidoc

@ph has a tool to extract this automatically, but it can be done manually if desired.