Doc: Add Fleet and Agent release notes for 8.19.13

[karenzone]

Content sourced from changelogs:

• Agent: Add Elastic Agent 8.19.13 release notes Add Elastic Agent 8.19.13 release notes elastic-agent#13141
• Fleet: Add Fleet Server 8.19.13 release notes Add Fleet Server 8.19.13 release notes fleet-server#6585

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Source PRs can be closed or merged.

PREVIEW: https://ingest-docs_bk_1898.docs-preview.app.elstc.co/guide/en/fleet/8.19/release-notes-8.19.13.html

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[karenzone]

Looks like we might have some PR links that don't belong. We're revisiting in source PRs:

• elastic/fleet-server#6585
• elastic/elastic-agent#13141

[jmikell821]

LGTM!