docs: Add cdk/listbox docs

Author: mmalerba

src/app/shared/documentation-items/documentation-items.ts

@@ -455,6 +455,14 @@ const DOCS: { [key: string]: DocItem[] } = {
         prefix: 'cdk-layout-',
       },
     },
+    {
+      id: 'listbox',
+      name: 'Listbox',
+      summary: 'Directives for building accessible listboxes.',
+      exampleSpecs: {
+        prefix: 'cdk-listbox-',
+      },
+    },
     {
       id: 'menu',
       name: 'Menu',