GH: Add action to build 9.x series plugin docs

[karenzone]

Related to #1826 with focus on validating new GH action without changing existing one.

[karenzone]

Testing and troubleshooting

Test1: Run new GH action for main.
Failed: https://github.com/elastic/logstash-docs/actions/runs/14254103345/job/39953351331#step:7:9

• Looks like the problem is at
  Line 58: bundle exec ruby plugindocs.rb --skip-existing --output-path ../logstash-docs ../logstash/plugins_version_docs.json.
  We still need to get to logstash repo for logstash/plugins_version_docs.json.

Look here: https://github.com/elastic/logstash/blob/eeb2162ae48e2abca8a59fb661f9e4542971abee/rakelib/plugins_docs_dependencies.rake#L21

[jsvd]

I have fixed the lack of the json file by reintroducing the cloning of the logstash repo. however #1817 was not backported to 9.0 so only main has docs/plugins/index.asciidoc