site-com/plugins/copy-md/index.ts

import path from 'node:path';
import fs from 'node:fs/promises';
import type {Plugin} from '@docusaurus/types';

type Options = {
  sourceDir: string;
  routeBasePath: string;
};

export default function copyMdPlugin(
  _context: unknown,
  options: Options,
): Plugin<void> {
  return {
    name: 'copy-md',
    async postBuild({siteDir, outDir}) {
      const src = path.join(siteDir, options.sourceDir);
      const dest = path.join(outDir, options.routeBasePath);
      await fs.mkdir(dest, {recursive: true});
      await copyMarkdownFiles(src, dest);
    },
  };
}

async function copyMarkdownFiles(src: string, dest: string): Promise<void> {
  const entries = await fs.readdir(src, {withFileTypes: true});
  for (const entry of entries) {
    const srcPath = path.join(src, entry.name);
    const destPath = path.join(dest, entry.name);
    if (entry.isDirectory()) {
      await fs.mkdir(destPath, {recursive: true});
      await copyMarkdownFiles(srcPath, destPath);
      continue;
    }
    if (entry.isFile() && /\.(md|mdx)$/i.test(entry.name)) {
      await fs.copyFile(srcPath, destPath);
    }
  }
}