docs(site-to-vpn): move page under Networks → Use Cases

The page describes a Networks-based setup; living under Network Routes
mis-categorised it.

- git mv to /manage/networks/use-cases/site-to-vpn.mdx (as a direct
  child of Use Cases, not under By Scenario)
- Navigation: remove entry from Network Routes → By Scenario, add under
  Networks → Use Cases
- Redirect old URL (/manage/network-routes/use-cases/by-scenario/site-to-vpn)
  to new URL, permanent
- Update in-tree links in use-cases/site-to-site/index.mdx and
  network-routes/use-cases/by-scenario/site-to-site-office.mdx

next.config.mjs

@@ -31,6 +31,11 @@ const nextConfig = {
                 destination: '/manage/networks',
                 permanent: true,
             },
+            {
+                source: '/manage/network-routes/use-cases/by-scenario/site-to-vpn',
+                destination: '/manage/networks/use-cases/site-to-vpn',
+                permanent: true,
+            },
             {
                 source: '/docs/getting-started/installation',
                 destination: '/get-started/install',