Make published Start pages discoverable and gate orphan routes

## Context

PR #508's documentation review found that `docs/site/src/content/docs/start/quickstart.mdx` is published but has no inbound documentation link or sidebar entry. The existing information-architecture test proves that configured sidebar slugs resolve, but it does not detect a published page that becomes orphaned. `credential-tour.mdx` is currently draft and is not part of the published-page problem.

This is useful documentation hardening, but it is not a correctness or security blocker for PR #508.

## Desired outcome

- Decide whether the published quickstart is a supported entry point, a redirect, or should become draft/removed.
- If supported, give it one intentional task-oriented navigation path without duplicating the generated first-country journey.
- Add a small reverse information-architecture gate that detects newly orphaned published pages while allowing an explicit, reviewed exception list for machine-only or redirect pages.
- Keep archived docsets and docset-relative links working.

## Definition of done

- Every published Start page is either intentionally reachable or explicitly exempted with rationale.
- A planted orphan-page negative control makes the architecture test fail.
- Current and archived documentation checks pass.
- No new onboarding path competes ambiguously with the primary journey.

Inizia da docs/site/src/content/docs/start/quickstart.mdx e dal test di information architecture esistente descritto nell'issue; esamina come sono rappresentate le pagine pubblicate, gli slug della sidebar, i docset archiviati e i link relativi. Decidi lo stato intenzionale e il percorso di navigazione del quickstart, quindi verifica che il controllo negativo di orphan inserito fallisca, mentre i controlli della documentazione corrente e archiviata abbiano esito positivo.

Scritto dal modello di indicizzazione a partire dal testo della issue.