Add link from APM Server to FIPS 8.19

[karenzone]

Add info/link from APM Server to FIPS Ingest topic.

PREVIEW: https://observability-docs_bk_4923.docs-preview.app.elstc.co/guide/en/observability/8.19/apm-securing-apm-server.html

Note to reviewers

I'll be out. Please feel free to merge when all checks have passed and the content meets your approval.

[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.}