docs: add Networks site-to-site use-case guide (#773)

* docs: add Networks site-to-site use-case guide

Add a canonical site-to-site guide built on Networks/Resources/Policies,
which is the recommended and actively developed approach. Reposition the
legacy Network Routes site-to-site Note to point at the new page and scope
Network Routes to the wide-open, no-policy case only. Add the new page to
the docs sidebar under Networks > Use Cases.

* docs: fix Resource fields, policy heading, and prerequisites in Networks site-to-site

- Remove non-existent 'Type: Subnet' field; describe Resource Groups under
  Additional Options to match the actual UI
- Rename Step 4 to 'Create access control policies'
- Trim prerequisites (drop account line and device examples)

* docs: clarify Masquerade requirement and Linux-only SNAT for site-to-site

src/components/NavigationDocs.jsx

@@ -215,6 +215,10 @@ export const docsNavigation = [
                 title: 'Site-to-VPN',
                 href: '/manage/networks/use-cases/site-to-vpn',
               },
+              {
+                title: 'Site-to-Site',
+                href: '/manage/networks/use-cases/site-to-site',
+              },
             ],
           },
         ],
@@ -464,15 +468,15 @@ export const docsNavigation = [
             links: [
               {
                 title: 'Getting Started',
-                href: '/manage/integrations/kubernetes'
+                href: '/manage/integrations/kubernetes',
               },
               {
                 title: 'Routing Peer',
-                href: '/manage/integrations/kubernetes/routing-peer'
+                href: '/manage/integrations/kubernetes/routing-peer',
               },
               {
                 title: 'Client Sidecar',
-                href: '/manage/integrations/kubernetes/client-sidecar'
+                href: '/manage/integrations/kubernetes/client-sidecar',
               },
               {
                 title: 'Gateway API',
@@ -1051,4 +1055,4 @@ function NavigationGroup({ group, className, hasChildren }) {
       </div>
     </li>
   )
-}
+}