[Observability] 9.5.4 release notes

[mdbirnstiehl]

Summary

Closes #8267 and adds the 9.5.4 Observability release notes.

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Tool(s) and model(s) used: Codex GPT 5.6 Luna

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• release-notes/elastic-observability/index.md

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Elastic Docs Style Checker (Vale)

Summary: 1 suggestion found

💡 Suggestions (1): Optional style improvements. Apply when helpful.

File	Line	Rule	Message
release-notes/elastic-observability/index.md	30	Elastic.FirstPerson	Use first-person pronouns sparingly. Rephrase 'me' if the sentence can focus on the user or product instead.

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