Add permanent redirect from /docs/installation to /docs/ (#155)

The installation content was moved to the docs root page. This redirect
ensures old links to /docs/installation still work.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: jackwildman

docs-site/src/app/installation/page.tsx

@@ -0,0 +1,5 @@
+import { permanentRedirect } from "next/navigation";
+
+export default function InstallationRedirect() {
+  permanentRedirect("/");
+}