Logstash plugin workflow: Update and follow a plugin doc through tooling

[karenzone]

Address this issue and follow plugin through bumping version, merging, publishing to rubygems, plugin doc generations, and publish.

Here's the issue we'll work through:
logstash-plugins/logstash-integration-kafka#196

[karenzone]

Process overview

1. Create a local workspace for logstash-plugins, and clone the plugin locally.

2. Update these files:

   • docs/index.asciidoc (with docs changes)
   • changelog.md (with description of changes, new version, and a link to the PR)
   • logstash--.gemspec (with the new version number)

3. Test it!
   We don't have any docs-ci or linting on plugin repos. The first docs validation happens after the PR is merged, the version is bumped, and the docs are generated. (@karenzone, link to How To Test docs.)

4. Put up a PR, and get it approved and merged.

5. Use the GitHub action in logstash-plugins to publish the plugin to rubygems.org. After the plugin has been published, Logstash plugin docs tooling takes over.

6. Within the next 6 hours, the Logstash Versioned Plugin job runs to pick up changes and create a PR in logstash-docs.

7. At Feature Freeze for the next release, we generate the stack-versioned plugin docs. For example, at 8.19.3, we'll generate plugin updates for the 8.19 branch. The generated output is added to a PR in the logstash-docs branch.

Asciidoc to MD conversion (9.0 and later only)

Up until this point, the Logstash plugin source files and generated files are all in Asciidoc. We have temporary tooling in place to take the asciidoc files from the logstash-docs repo, convert them to MD, and output them to logstash-docs-md.