docs: add applies_to frontmatter for docs v3 cumulative docs

[alexandra5000]

Summary

Adds mandatory applies_to frontmatter to all reference documentation pages to align with the Docs versioning contributor guidelines and docs v3.

Tags are consistent with page/section content.

Relates to #256

Generative AI disclosure

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

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Claude Sonnet 4.5 using Cursor to double-check the updated files, write PR description

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[github-actions]

🔍 Preview links for changed docs

• docs/reference/_elasticsearch_security.md
• docs/reference/_enrichers_2.md
• docs/reference/_extending_ecsdocument.md
• docs/reference/_formatters.md
• docs/reference/_usage.md
• docs/reference/apm-nlog-enricher.md
• docs/reference/apm-serilog-enricher.md
• docs/reference/benchmark-dotnet-data-shipper.md
• docs/reference/data-shippers.md
• docs/reference/ecs-dotnet.md

More links …

• docs/reference/ecs-ingest-channels.md
• docs/reference/extensions-logging-data-shipper.md
• docs/reference/index.md
• docs/reference/intro_to_xyz.md
• docs/reference/log4net-formatter.md
• docs/reference/nlog-formatter.md
• docs/reference/serilog-data-shipper.md
• docs/reference/serilog-formatter.md
• docs/reference/setup.md

[florent-leborgne]

@alexandra5000 Can you double check this against our dimension tagging guidelines (https://www.elastic.co/docs/contribute-docs/how-to/cumulative-docs/guidelines#dimensions); also summarized here on Slack (docs internal)?

[alexandra5000]

@florent-leborgne Ah, I see what I did wrong there. Thanks Florent!