[Observability] 9.5 release notes

[mdbirnstiehl]

Summary

This PR closes #7525 and adds the Observability release notes for the 9.5 release.

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🔍 Preview links for changed docs

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

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Elastic Docs Style Checker (Vale)

Summary: 2 warnings, 3 suggestions found

⚠️ Warnings (2): Fix when the suggestion improves clarity or correctness.

File	Line	Rule	Message
release-notes/elastic-observability/index.md	56	Elastic.Spelling	'minimap' is a possible misspelling.
release-notes/elastic-observability/index.md	138	Elastic.Spelling	'untracking' is a possible misspelling.

💡 Suggestions (3): Optional style improvements. Apply when helpful.

File	Line	Rule	Message
release-notes/elastic-observability/index.md	140	Elastic.Clone	Use cloning only when referring to cloning a GitHub repository or creating a copy that is linked to the original. Often confused with 'copy' and 'duplicate'.
release-notes/elastic-observability/index.md	143	Elastic.WordChoice	Consider using 'turn off, silence' instead of 'mute', unless the term is in the UI.
release-notes/elastic-observability/index.md	143	Elastic.WordChoice	Consider using 'turn off, silence' instead of 'mute', unless the term is in the UI.

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[florent-leborgne]

LGTM from what I read. There is 1 breaking change and 1 deprecation entry that are already covered in the Kibana release notes (I think you can drop them, or maybe add a link at the top of all release notes to also check the kibana ones?)

Also, similarly to a comment I made on the Security release notes, we might start to categorize the entries here too (Streams, Synthetics, etc. etc.)

[alisonelizabeth]

approved release note related to frozen phase enhancement