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.

Start with docs/site/src/content/docs/start/quickstart.mdx and the existing information-architecture test described in the issue; inspect how published pages, sidebar slugs, archived docsets, and relative links are represented. Decide the quickstart's intentional status and navigation path, then verify the planted orphan negative control fails while current and archived documentation checks pass.

Written by the indexing model from the issue text.