fix: resolve index pages to parent path for .md URLs

encyclopedia/index.mdx now outputs encyclopedia.md instead of
encyclopedia/index.md, matching how Docusaurus routes index pages.

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

Author: lennessyy

plugins/markdown-pages/index.js

@@ -25,7 +25,8 @@ module.exports = function markdownPagesPlugin(context, options = {}) {
     const withoutExt = rel.replace(/\.(md|mdx)$/i, '');
     const id = frontmatter.id || path.basename(withoutExt);
     const dir = path.dirname(withoutExt);
-    if (dir === '.') return id;
+    if (dir === '.') return id === 'index' ? 'index' : id;
+    if (id === 'index') return dir;
     return `${dir}/${id}`;
   }