Doc: Add page-level applies_to tags to Logstash docs (Group 1)

[karenzone]

Summary

Group 1: Getting Started, Installation, and Setup (~20 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

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)
• run exhaustive tests : Run the exhaustive tests Buildkite pipeline.

[github-actions]

✅ Vale Linting Results

No issues found on modified lines!

---

The Vale linter checks documentation changes against the Elastic Docs style guide.

To use Vale locally or report issues, refer to Elastic style guide for Vale.

[github-actions]

🔍 Preview links for changed docs

• docs/reference/connecting-to-cloud.md
• docs/reference/connecting-to-serverless.md
• docs/reference/dir-layout.md
• docs/reference/docker-config.md
• docs/reference/docker.md
• docs/reference/first-event.md
• docs/reference/getting-started-with-logstash.md
• docs/reference/how-logstash-works.md
• docs/reference/index.md
• docs/reference/installing-logstash.md

More links …

• docs/reference/running-logstash-command-line.md
• docs/reference/running-logstash-kubernetes.md
• docs/reference/running-logstash-windows.md
• docs/reference/running-logstash.md
• docs/reference/setting-up-running-logstash.md
• docs/reference/shutdown.md
• docs/reference/upgrading-logstash.md
• docs/reference/upgrading-minor-versions.md
• docs/reference/upgrading-using-direct-download.md
• docs/reference/upgrading-using-package-managers.md