Revisit and evaluate skiplists

[karenzone]

We have skiplists in three locations:

• VPR: https://github.com/elastic/docs-tools/blob/master/versioned_plugins.rb [1]
• LSR: https://github.com/elastic/logstash/blob/master/rakelib/plugins-metadata.json [2]
• Another skiplist: https://github.com/elastic/docs-tools/blob/master/settings.yml [3]

Comments:

• [3] lives in docs-tools, and skips whole repos. (If we're skipping an entire repo, will adding repos here be all we need to skip a plugin?)
• [2] contains settings other than "skip-list": "default-plugins" and sometimes "core-specs"
• Do we have use cases where we want to skip a plugin for one but not both of Logstash Reference and Versioned Plugin Reference?

ToDo:

• Figure out how these skiplists interrelate
• See if we can simplify
• Document best practices:
  • https://github.com/elastic/ingest-dev/tree/master/logstash/logstash-docs-processes
  • comments in code