Doc: Add page-level applies_to tags to Logstash content (Group 4)

[karenzone]

Summary

Group 4: Plugins, Integrations, Performance, and Best Practices (~18 files)
Adds applies_to tags to show applicability for each topic.

Related: https://github.com/elastic/docs-content-internal/issues/260

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
  Tool(s) and model(s) used: Cursor and claude-4.6-opus-high
• No

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🔍 Preview links for changed docs

• docs/reference/deploying-scaling-logstash.md
• docs/reference/logstash-to-logstash-communications.md
• docs/reference/ls-to-ls-http.md
• docs/reference/ls-to-ls-lumberjack.md
• docs/reference/ls-to-ls-native.md
• docs/reference/offline-plugins.md
• docs/reference/performance-troubleshooting.md
• docs/reference/performance-tuning.md
• docs/reference/plugin-concepts.md
• docs/reference/plugin-generator.md

More links …

• docs/reference/private-rubygem.md
• docs/reference/tips-best-practices.md
• docs/reference/tuning-logstash.md
• docs/reference/use-filebeat-modules-kafka.md
• docs/reference/use-ingest-pipelines.md
• docs/reference/working-with-filebeat-modules.md
• docs/reference/working-with-plugins.md
• docs/reference/working-with-winlogbeat-modules.md