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

[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.

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[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

[github-actions]

Warning

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.

[mergify]

This pull request is now in conflict. Could you fix it @kilfoyle? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch <remote-repo>
git checkout -b 8.18.0-rn <remote-repo>/8.18.0-rn
git merge <remote-repo>/main
git push <remote-repo> 8.18.0-rn