Generate the 9.0.4 release notes for Observability, and publish them before or on the release day.

• Feature freeze: Tue, July 8 2025
• Merge release notes by: Mon, July 14 2025
• Anticipated release date: Tue, July 15 2025
• Docs release issue: https://github.com/elastic/dev/issues/3210

To-do

• Draft the release notes
• Add the release notes PR to the docs release issue
• Known issues:
  • Carry forward known issues that haven't been fixed yet
  • Update the summaries of known issues that were fixed in the current release (add fix details): [Internal]: Known issue for Assistant knowledge base setting @ 9.0.3 #1895 (comment)
  • Document new known issues
• Merge the Observability release notes PR