docs: add Commercial License Overview nav entry (#892)

Turn "Commercial License" into a plain nav group and add an "Overview"
child pointing at /selfhosted/enterprise, above "Getting Started". This
matches the pattern already used by Networks, Cloud Marketplaces,
Observability, and Troubleshooting, where the section index page is
reachable as its own "Overview" link rather than only via the group
label.

src/components/NavigationDocs.jsx

@@ -550,9 +550,12 @@ export const docsNavigation = [
             { title: 'Automated Setup', href: '/selfhosted/automated-setup' },
             {
                 title: 'Commercial License',
-                href: '/selfhosted/enterprise',
                 isOpen: false,
                 links: [
+                    {
+                        title: 'Overview',
+                        href: '/selfhosted/enterprise',
+                    },
                     {
                         title: 'Getting Started',
                         href: '/selfhosted/enterprise/getting-started',