fix: move /en/config.ts to /config.ts, allow root-level configuration overloading

Author: zhangyx1998

docs/en/config.ts renamed to docs/config.ts

@@ -99,6 +99,7 @@ function getConfigLayers(root: SiteData, path: string): SiteData[] {
     if (key in additionalConfig) configs.push(additionalConfig[key] as SiteData)
     segments.pop()
   }
+  if ('/' in additionalConfig) configs.push(additionalConfig['/'] as SiteData)
   return [...configs, root]
 }