[9.2] (backport #18859) Doc: Add page-level applies_to tags to Logstash content (Group 2)

[mergify]

Summary

Group 2: Pipeline Configuration and Data Processing (~26 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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This is an automatic backport of pull request #18859 done by [Mergify](https://mergify.com).

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🔍 Preview links for changed docs

• docs/reference/advanced-logstash-configurations.md
• docs/reference/advanced-pipeline.md
• docs/reference/config-examples.md
• docs/reference/config-setting-files.md
• docs/reference/configuration-file-structure.md
• docs/reference/creating-logstash-pipeline.md
• docs/reference/data-deserialization.md
• docs/reference/dead-letter-queues.md
• docs/reference/ecs-ls.md
• docs/reference/environment-variables.md

More links …

• docs/reference/event-api.md
• docs/reference/event-dependent-configuration.md
• docs/reference/execution-model.md
• docs/reference/field-extraction.md
• docs/reference/glob-support.md
• docs/reference/lookup-enrichment.md
• docs/reference/memory-queue.md
• docs/reference/multiline.md
• docs/reference/multiple-input-output-plugins.md
• docs/reference/multiple-pipelines.md
• docs/reference/persistent-queues.md
• docs/reference/pipeline-to-pipeline.md
• docs/reference/processing.md
• docs/reference/queues-data-resiliency.md
• docs/reference/reloading-config.md
• docs/reference/transforming-data.md