Add Fleet & Agent 8.18.0 Release Notes

[kilfoyle]

This adds the 8.18.0 Fleet & Elastic Agent Release Notes:

• Fleet contents in Kibana Release Notes PR - TBD
• Fleet Server contents in BC5 changelog - TBD
• Elastic Agent contents from BC5 changelog - TBD

See preview page
Closes: #1654

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[github-actions]

It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.

[karenzone]

LGTM