fix(docs): add summarizing storage page to the Swift sidebar (#646)

The page shipped ~July 14 without a sidebar entry; the
starlight-sidebar-topics middleware throws for any page not attached to
a topic, which has failed every docs deploy since — the live site is
two weeks stale. Verified locally: full astro build now completes (111
pages).

Closes #645

Code written by Claude (Fable 5), architected and approved by @cornelcroi.

Author: cornelcroi

docs/astro.config.mjs

@@ -250,6 +250,7 @@ export default defineConfig({
 											{ label: 'File', slug: 'swift/storage/built-in/file' },
 											{ label: 'Device (SwiftData)', slug: 'swift/storage/built-in/device' },
 											{ label: 'Transforming (PII scrub)', slug: 'swift/storage/built-in/transforming' },
+											{ label: 'Summarizing', slug: 'swift/storage/built-in/summarizing' },
 										],
 									},
 									{ label: 'Custom store', slug: 'swift/storage/custom' },