docs: restore 1 dead link(s) via Wayback

[mirkosalvato1-ctrl]

This PR fixes 1 broken outbound documentation link(s).

Changes:

• docs/architecture.md: https://gateway-api-inference-extension.sigs.k8s.io/guides/serving-mul → http://web.archive.org/web/20260521045059/https://gateway-api-inferenc (Wayback snapshot (host dead))

Fixes #956

[github-actions]

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[elevran]

@mirkosalvato1-ctrl thank you for the PR, fixing a broken link using Wayback machine.
A few requests:

• please sign your commits: per developer documentation git commit -S is required for DCO to pass (adds Signed-off-by line)
• it is unclear why this PR Fixes #956 - they seem mostly unrelated and [RFC][CI]: CI/CD Improvements for llm-d-router #956 is much broader in scope. For small fixes (such as typos, minor documentation changes, etc.) we do not require an explicit open issue.
• prefer that the broken link is made live by referring to the new relevant documentation location under llm-d