docs(admin): add WHP super-admin section (5 draft articles)

Adds /whp/admin/ with: overview, server-settings, coraza-waf,
site-monitoring, user-management. Articles are product-neutral
(no VDS in titles or required framing) since super admin may be
offered as a separate service later. They're marked Draft via:

- 'Draft' sidebar badge on the group + each article
- A draft callout partial at the top of every page noting WIP

The super-admin-callout partial (renamed from vds-only-callout)
mentions VDS as today's most common way to get super admin without
locking the docs to that single product.

astro.config.mjs

@@ -53,6 +53,11 @@ export default defineConfig({
               label: 'Add-ons',
               items: [{ autogenerate: { directory: 'whp/add-ons' } }],
             },
+            {
+              label: 'Admin',
+              badge: { text: 'Draft', variant: 'caution' },
+              items: [{ autogenerate: { directory: 'whp/admin' } }],
+            },
           ],
         },
         // Future products only appear once they have content.