Point users to where the template lives

I was going to submit a PR for this but (I'll openly admit) I wasn't sure on how best to do this given we are dealing with 3 versions of ES, and thus, the related docs.

Should we just link to https://github.com/logstash-plugins/logstash-output-elasticsearch/tree/master/lib/logstash/outputs/elasticsearch and then let users find there way, or link to specific versions of the json from the version specific doc page?

Start with the Elasticsearch output plugin directory at lib/logstash/outputs/elasticsearch and the version-specific documentation pages mentioned in the issue. Compare how the three ES versions expose their JSON templates and determine the appropriate linking approach. Done means users can reach the relevant template location from the version-specific documentation.

Written by the indexing model from the issue text.