[신규 번역] Part 3. 3.6 Fetch API 번역

### 작업할 내용
- 원문 저장소 : https://github.com/javascript-tutorial/en.javascript.info/blob/master/5-network/06-fetch-api/article.md
- KO 번역 저장소 : https://github.com/javascript-tutorial/ko.javascript.info/blob/master/5-network/06-fetch-api/article.md
