feat: Add robots.txt to let google know our sitemap

[mark86092]

Close #1387

這個 PR 是？

將 robots.txt 放到 pubic 資料夾，會當作一般的 static file serve

我應該如何手動測試？

• [ ] 嘗試 npm run build，檢查 build/public/，如下

ls -al build/public/
total 56
drwxr-xr-x 3 ycchen ycchen  4096 Aug  2 14:11 .
drwxr-xr-x 3 ycchen ycchen 24576 Aug  2 14:11 ..
-rw-r--r-- 1 ycchen ycchen 16958 Aug  2 14:11 favicon.ico
-rw-r--r-- 1 ycchen ycchen    76 Aug  2 14:11 robots.txt
drwxr-xr-x 5 ycchen ycchen  4096 Aug  2 14:11 static