Validate doc changes (asciidoc formatting, cross-document links) for plugin doc source

The `logstash` and `logstash-docs` repos now have the ability to do a complete doc build and throw errors if there are asciidoc formatting errors, bad cross-document links, etc. Our plugin docs are spread out in repos throughout the `logstash-plugins` org. Any doc errors don't manifest until we generate and transform the plugin docs, and they land in `logstash-docs`.

Could we validate doc changes sooner?

This enhancement has been needed for a long time. See
[Create infrastructure for testing plugin docs for a single plugin #19](https://github.com/elastic/docs-tools/issues/19). It was closed due to inactivity, not because it was resolved.

UPDATED:
~This issue is especially painful because of a regression that passes a bad Logstash Versioned Plugin Reference to the elastic doc build and then crashes it: #51~ Thankfully, this issue has been resolved.