Write documentation for the Github integration flow

# Problem

We need big picture "how it works" documentation for github integration flow and what it does.

A big chunk of it is explained in #1331 and some of it is additionally explained in #1370

However, it needs to be written in an easier to digest format for developers to refer to when working on integration features.

先閱讀 issue #1331 和 #1370，其中包含現有的 GitHub integration flow 說明。將這些內容整理成一份更容易理解的開發者參考文件，涵蓋該 flow 的運作方式及其作用。當 integration behavior 的文件清楚到足以讓負責開發 integration features 的開發者據此工作時，即表示完成。

由索引模型根據 Issue 內容生成。