[Observability] 9.1.1 release notes

[mdbirnstiehl]

Generate the 9.1.1 release notes for Observability, and publish them before or on the release day.

• Feature freeze: August 4
• Merge release notes by: August 6
• Anticipated release date: August 7
• Docs release issue: https://github.com/elastic/dev/issues/3242