Add documentation for backend development (#17512)

* Add documentation for backend development

* Update backend guidline

* More sections

* Add modules/setting and modules/git

* Uniform gitea as Gitea

* some improvements

* some improvements

docs/content/doc/developers.zh-cn.md

@@ -0,0 +1,13 @@
+---
+date: "2016-12-01T16:00:00+02:00"
+title: "开发者"
+slug: "developers"
+weight: 40
+toc: false
+draft: false
+menu:
+  sidebar:
+    name: "开发者"
+    weight: 50
+    identifier: "developers"
+---