Add basePath for GitHub Pages subpath deployment

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mokuwaki

next.config.ts

@@ -2,6 +2,7 @@ import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
   output: "export",
+  basePath: "/rollberry-docs",
   images: { unoptimized: true },
   reactCompiler: true,
 };