Doc: Add Fleet Server and Elastic Agent release notes for 8.18.7

[karenzone]

Adds Fleet Server and Elastic Agent release notes for 8.18.7

Related:

• Fleet and Agent 8.18.7 release notes docs-content#2910

Source PRs:

• Changelog 8.18.7 elastic-agent#9922
• Changelog for 8.18.7 fleet-server#5507

No forwardports required.

[github-actions]

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[karenzone]

PR previews aren't working, so here's a visual:

[colleenmcginnis]

As long as the build succeeds, this looks good to me!