Our blog should live at DOMAIN/blog not DOMAIN/category/blog

I want this change mostly for minor aesthetic reasons. But I think the existence of the "blog" category is a clear sign that whoever first set the site up didn't understand Jekyll collections to (1) exist, and (2) be great. Both 1 and 2 are true of me, so I'd like a more-reasonable setup for the blog and any other content-types we're looking have later.

先檢查現有的 Jekyll 分類與部落格 URL 結構，然後檢視網站目前如何組織內容。定義一個基於 collection 的設定，讓部落格可在 DOMAIN/blog 提供服務，並支援未來的內容類型；當新的 URL 不含分類路徑即可正常運作，且組織方式已完成文件化時，即視為完成。

由索引模型根據 Issue 內容生成。