[Logstash API] Complete API doc migration to OpenAPI spec

[karenzone]

Description: Complete Logstash API migration to OpenAPI Spec (OAS), and publish API docs on bump.sh
Timeline: Release 9.0.0
Status: Basic structure and framework are in place. Need to QA existing content and add missing content.
More details: https://github.com/elastic/ingest-dev/issues/4861

API	Current output location (Source)	New output location (Source)	Projected scope
Intro	Monitoring Logstash with APIs	Logstash APIs	QA
Health report	Health report API	Health report	QA and add missing content
Hot threads	Hot Threads API	Hot threads	QA
Node info	Node info API	Node info	QA
Node stats	Node Status API	Node stats	QA
Plugins info	Plugins info API	Plugins info	QA

OpenAPI docs are NOT subject to docs feature freeze, so full speed ahead!

[karenzone]

@robbavey,here's the issue you requested.