feat: expose CDK accordion docs

Follow-up to angular/components#22829. Exposes the CDK accordion docs in the navigation.

Author: crisbeto

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

@@ -391,6 +391,14 @@ const DOCS: { [key: string]: DocItem[] } = {
         prefix: 'focus-monitor-',
       },
     },
+    {
+      id: 'accordion',
+      name: 'Accordion',
+      summary: 'Component with one or more expandable sections.',
+      exampleSpecs: {
+        prefix: 'cdk-accordion-',
+      },
+    },
     {
       id: 'bidi',
       name: 'Bidirectionality',