Fix dead links on migrated website (Error 404 etc)

We know about a few problem links e.g. #8 #13 #17, but should check this systematically. There are also some external links which no longer work.

Ideally we'd have a TravisCI configuration to run Jekyll and then spider the `_site/` folder to spot any link breakage.

從 issues #8、#13 和 #17 中已知問題的連結開始，並檢視 Jekyll 如何產生 `_site/` 資料夾。確定 TravisCI 設定如何建置網站並爬取內部與外部連結。完成的標準是現有的失效連結已獲得修正，而且自動檢查能夠偵測未來的連結故障。

由索引模型根據 Issue 內容生成。