Doc: Research and scrub book-scoped variables for Logstash Reference

[karenzone]

Docs pre-migration task

The concept of book-scoped variables goes away with our migration to the new MDX-based docs system.
Removing book-scoped variables from the Logstash Reference is a pre-migration task.

Source file: https://github.com/elastic/logstash/blame/main/docs/index.asciidoc#L9-L22

Related:

• PR for Elasticsearch Reference: [DOCS] Refactor book-scoped variables in docs/reference/index.asciidoc elasticsearch#107413
• Issue for Versioned Plugin Reference: [Doc: Research and scrub book-scoped variables for Logstash Versioned Plugin Reference #16081 ]