code: function registerPartials (folder) {
    const partialsFolder = path.join(folder, '_includes')
  
    const partials = fs.readdirSync(partialsFolder)
      .filter(isValidPartial)
      .reduce((result, partial) => {
        const ext = path.extname(partial)
        const fileFullPath = path.join(partialsFolder, partial)
        const data = fs.readFileSync(fileFullPath, 'utf-8')
  
        // Store as `"filename without extension": content`.
        result[path.basename(partial, ext)] = data
        return result
      }, {})
  
    handlebars.registerPartial(partials)
  }
docs: Add all the required partials.