🐛 fix: point top-nav "KubeStellar" entry to working architecture page (#1454)

The top-nav dropdown (rendered from public/config/shared.json on every
docs page) linked to `/docs/what-is-kubestellar/overview`, but
`what-is-kubestellar` is not a registered project in the catch-all
`/docs/[...slug]` route (see `src/app/docs/[...slug]/page.tsx` —
`getProjectFromSlug` only knows `a2a`, `kubeflex`, `multi-plugin`,
`kubestellar-mcp`, `console`, and the default `kubestellar`). No
`what-is-kubestellar/overview.md` file exists anywhere under
`docs/content/`, so the page always 404s — broken-link checker flagged
it at hash 7bd42cdc while crawling /docs/multi-plugin.

Repointed to `/docs/kubestellar/architecture`, which resolves to the
existing `docs/content/kubestellar/architecture.md` and whose content
("KubeStellar provides multi-cluster deployment of Kubernetes objects")
matches the shared.json description "Multi-cluster configuration
management".

Related broken `what-is-kubestellar/*` references remain in Footer.tsx,
DocsSidebar.tsx, GetStartedSection.tsx, and products/page.tsx. Those
weren't in scope for the specific link hash reported in #1453 but
should be swept in a follow-up (they'll trigger their own link-checker
issues soon enough).

Fixes #1453

Signed-off-by: Andy Anderson <andy@clubanderson.com>

Author: clubanderson

public/config/shared.json

@@ -226,7 +226,7 @@
     },
     {
       "title": "KubeStellar",
-      "href": "/docs/what-is-kubestellar/overview",
+      "href": "/docs/kubestellar/architecture",
       "description": "Multi-cluster configuration management",
       "secondary": true
     },