[8.8] [DOCS] Adds 8.8.0 release notes (#155951) (#157420)

# Backport

This will backport the following commits from `main` to `8.8`:
- [[DOCS] Adds 8.8.0 release notes
(#155951)](#155951)

<!--- Backport version: 8.9.7 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

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Co-authored-by: Kaarina Tungseth <kaarina.tungseth@elastic.co>