docs-site: serve under /ktx and redirect ktx.sh to docs.kaelio.com

- Add Next.js basePath '/ktx' so the docs site mounts under /ktx,
  enabling it to be reverse-proxied by docs.kaelio.com.
- Add host-based redirects so docs.ktx.sh and ktx.sh permanently
  redirect to https://docs.kaelio.com/ktx, making docs.kaelio.com
  the single canonical home for KTX docs.
- Update markdown-preference middleware to be basePath-aware so the
  llms.mdx rewrite still fires on /ktx/docs/* requests.

docs-site/middleware.ts

@@ -12,12 +12,13 @@ export function middleware(request: NextRequest) {
   }
 
   const { pathname } = request.nextUrl;
-  if (!pathname.startsWith("/docs/") || pathname.endsWith(".md")) {
+  const docsIndex = pathname.indexOf("/docs/");
+  if (docsIndex < 0 || pathname.endsWith(".md")) {
     return NextResponse.next();
   }
 
   const rewriteUrl = request.nextUrl.clone();
-  rewriteUrl.pathname = `/llms.mdx${pathname}`;
+  rewriteUrl.pathname = `${pathname.slice(0, docsIndex)}/llms.mdx${pathname.slice(docsIndex)}`;
 
   return NextResponse.rewrite(rewriteUrl);
 }