DOC-3261: user defined systemd extensions

[Linus-SpectroCloud]

✅ Pull Request Checklist

Checklist

• The PR description in the Describe the Change section explains what changed and provides any additional context.
• A Jira ticket is provided in the Jira Tickets section AND in the PR title.
• Preview links are for all affected pages are provided in the Changed Pages.
• Conduct a self-review for your pages using your preview links. Verify grammar, clarity, and accuracy.
• Check formatting for your pages
  • Verify indentations, admonitions, and tables (if applicable).
  • Verify all images display. No images on this page.
• Ensure all Vale comments are resolved. 0 errors.
• All CI jobs have completed successfully.
• Add Backport labels if the change should be reflected in previous versions. Add auto-backport + backport-version-4-10 (systemd extensions is a 4.10.a feature).
• Outside review: Does this PR require review outside of Docs? Yes — blocked on Engineering (Chris Paap) to confirm the extension declaration schema before the stub sections can be filled.
• Add the notify-slack label once this checklist has been completed. Draft / WIP: do not merge until the stubbed sections are filled in.

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📝 Describe the Change

This PR adds a new sibling page, Add User-Defined systemd Extensions, under the Edge provider-image workflow. It documents how a customer builds their own systemd extension and adds it to a Palette Edge cluster to run custom software on the host, such as an endpoint security agent or a VPN client. This is the DOC-3261 GA-exit-criterion requested in Chris Paap's systemd-extensions review, and is a companion to the existing Deliver Kubernetes and Agent Binaries via systemd Extensions page.

This is a scaffold held as a Draft. The structure and all the content that does not depend on the declaration schema are complete; the schema-dependent sections are stubbed with TODO comments pending an Engineering answer.

Complete:

• Intro — what user-defined extensions are and when to use them, with linked first mentions of CrowdStrike Falcon and Tailscale.
• Support Requirements — cross-references the delivery page's requirements rather than duplicating them.
• Build the Extension — points to the upstream Kairos sys-extensions guide and documents the extension-release.d metadata requirement.
• Supply Configuration and Secrets — the keep-secrets-out-of-the-read-only-image and supply-via-cloud-config guidance.

Stubbed, pending Engineering confirmation (Chris Paap):

• Add the Extension to a Cluster — the supported declaration path and exact YAML schema (stylus.extensions vs. a custom pack, exact fields, supported source schemes).
• The cloud-config example.
• Examples — CrowdStrike Falcon and Tailscale worked examples.

Once the schema is confirmed, those sections fill in and the PR comes out of Draft.

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💻 Changed Pages

• Add User-Defined systemd Extensions

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🎫 Jira Tickets

• DOC-3261 — Document building and adding user-defined systemd extensions to Palette Edge clusters

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