docs: document admin environment variables (#11313)

www/apps/book/sidebar.mjs

@@ -456,6 +456,11 @@ export const sidebar = sidebarAttachHrefCommonOptions([
             path: "/learn/fundamentals/admin/ui-routes",
             title: "Admin UI Routes",
           },
+          {
+            type: "link",
+            path: "/learn/fundamentals/admin/environment-variables",
+            title: "Environment Variables",
+          },
           {
             type: "link",
             path: "/learn/fundamentals/admin/constraints",