function mapSubDoc(subDoc, options, exportsObj) {
  const dirName = `docs/${subDoc}`;

  const files = fs.readdirSync(dirName).filter(file => file.endsWith('.md'));

  files.forEach((filename) => {
    const content = fs.readFileSync(`${dirName}/${filename}`, 'utf8');
    exportsObj[`${dirName}/${filename}`] = {
      ...options,
      title: `${options.title} ${content.split('\n')[0].replace(/^#+/, '').trim()}`
    };
  });
}
Map sub-directories with custom options
@param {String} subDoc Path to the subdoc, like "tutorials" (no beginning or ending slash)
@param {DocsOptions} options Options applied to all files in the subdoc (title gets appended to provided title)
@param {Object} exportsObj The "module.exports" object to apply changes to