chore(docs): bump dependencies and adapt to fumadocs-core 16.15.11

Updates the docs site's dependencies, including the Fumadocs packages,
Next 16.3.5, React 19.3 and three majors: mermaid 12, vitest 5 and
pnpm 12. Two code changes follow from the bump:

- fumadocs-core 16.15.11 makes `llms().index()` return a Promise, so
  the llms.txt route now awaits it; tsc rejected the old synchronous
  call
- lucide-react 1.46 renamed the BookMarked icon to BookBookmark. The
  old name is still exported, but lucideIconsPlugin looks names up in
  lucide's `icons` map, which only has the new one, so the Reference
  section lost its sidebar icon in all four locales. The build only
  printed a warning.

minimumReleaseAgeExclude gains entries for the newly installed
versions.

Checked with typecheck, lint, vitest (106 tests) and a full build: no
plugin warnings, and each locale's rendered /docs page contains the
book-bookmark icon.

docs/app/llms.txt/route.ts

@@ -3,6 +3,6 @@ import { llms } from 'fumadocs-core/source';
 
 export const revalidate = false;
 
-export function GET() {
-  return new Response(llms(source).index());
+export async function GET() {
+  return new Response(await llms(source).index());
 }