static _generateForIndex(config) {
    let indexContent = '';

    if (fs.existsSync(config.index)) {
      indexContent = fs.readFileSync(config.index, {encode: 'utf8'}).toString();
    } else {
      console.log(`[31mwarning: ${config.index} is not found. Please check config.index.[0m`);
    }

    const tag = {
      kind: 'index',
      content: indexContent,
      longname: path.resolve(config.index),
      name: config.index,
      static: true,
      access: 'public'
    };

    return tag;
  }
generate index doc
@param {ESDocConfig} config
@returns {Tag}
@private