Doc: Keep generated plugin docs aligned pending updated migration strategy - Sprint 2

[karenzone]

Keep plugin docs updated in logstash-docs repo so that we don't lose content when we do migration updates.
Benefits:

• Keeps content current so that it's ready for re-migration
• Exposes any formatting errors in source docs so that we don't have any problems hiding from us.

[karenzone]

Related

• https://github.com/elastic/logstash-docs-md/issues/19
• https://github.com/elastic/logstash-docs-md/issues/4 (list is no longer being updated)

Fixes to keep content accurate and flowing

• auto generated update of versioned plugin documentation with corrections logstash-docs#1860