docs: surface exit nodes under Routes and document broad-access mitigations

Exit nodes previously lived only under Use Cases. Add them to the Routes
sidebar group and mention them on the Routes overview. Document that a
default route grants access to everything the routing peer can reach,
with Block LAN access and network isolation as mitigations, and link
How Routing Peers Work back to the exit nodes guide.

src/components/NavigationDocs.jsx

@@ -227,6 +227,10 @@ export const docsNavigation = [
                         title: 'Site-to-Site (legacy)',
                         href: '/manage/network-routes/use-cases/site-to-site',
                     },
+                    {
+                        title: 'Exit Nodes',
+                        href: '/use-cases/remote-access/exit-nodes',
+                    },
                     {
                         title: 'Access Control',
                         href: '/manage/network-routes/access-control',