[DOCS] Adds 8.8.0 release notes (elastic#155951)

## Summary

Adds the release notes for the 8.8.0 release.

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Co-authored-by: Lisa Cawley <lcawley@elastic.co>
Co-authored-by: Jen Huang <its.jenetic@gmail.com>
Co-authored-by: Larry Gregory <lgregorydev@gmail.com>
Co-authored-by: Alison Goryachev <alisonmllr20@gmail.com>
Co-authored-by: István Zoltán Szabó <istvan.szabo@elastic.co>
Co-authored-by: Devon Thomson <devon.thomson@elastic.co>