remove docs dir (#1786)

the post doc move https://gin-gonic.com/blog/

docs/how-to-build-an-effective-middleware.md

@@ -1,137 +0,0 @@
-# How to build one effective middleware?
-
-## Consitituent part
-
-The middleware has two parts:
-
-  - part one is what is executed once, when you initialize your middleware. That's where you set up all the global objects, logicals etc. Everything that happens one per application lifetime.
-
-  - part two is what executes on every request. For example, a database middleware you simply inject your "global" database object into the context. Once it's inside the context, you can retrieve it from within other middlewares and your handler function.
-
-```go
-func funcName(params string) gin.HandlerFunc {
-    // <---
-    // This is part one
-    // --->
-    // The follow code is an example
-    if err := check(params); err != nil {
-        panic(err)
-    }
-
-    return func(c *gin.Context) {
-        // <---
-        // This is part two
-        // --->
-        // The follow code is an example
-        c.Set("TestVar", params)
-        c.Next()    
-    }
-}
-```
-
-## Execution process
-
-Firstly, we have the follow example code:
-
-```go
-func main() {
-	router := gin.Default()
-
-	router.Use(globalMiddleware())
-
-	router.GET("/rest/n/api/*some", mid1(), mid2(), handler)
-
-	router.Run()
-}
-
-func globalMiddleware() gin.HandlerFunc {
-	fmt.Println("globalMiddleware...1")
-
-	return func(c *gin.Context) {
-		fmt.Println("globalMiddleware...2")
-		c.Next()
-		fmt.Println("globalMiddleware...3")
-	}
-}
-
-func handler(c *gin.Context) {
-	fmt.Println("exec handler.")
-}
-
-func mid1() gin.HandlerFunc {
-	fmt.Println("mid1...1")
-
-	return func(c *gin.Context) {
-
-		fmt.Println("mid1...2")
-		c.Next()
-		fmt.Println("mid1...3")
-	}
-}
-
-func mid2() gin.HandlerFunc {
-	fmt.Println("mid2...1")
-
-	return func(c *gin.Context) {
-		fmt.Println("mid2...2")
-		c.Next()
-		fmt.Println("mid2...3")
-	}
-}
-```
-
-According to [Consitituent part](#consitituent-part) said, when we run the gin process, **part one** will execute firstly and will print the follow information:
-
-```go
-globalMiddleware...1
-mid1...1
-mid2...1
-```
-
-And init order are:
-
-```go
-globalMiddleware...1
-    |
-    v
-mid1...1
-    |
-    v
-mid2...1
-```
-
-When we curl one request `curl -v localhost:8080/rest/n/api/some`, **part two** will execute their middleware and output the following information:
-
-```go
-globalMiddleware...2
-mid1...2
-mid2...2
-exec handler.
-mid2...3
-mid1...3
-globalMiddleware...3
-```
-
-In other words, run order are:
-
-```go
-globalMiddleware...2
-    |
-    v
-mid1...2
-    |
-    v
-mid2...2
-    |
-    v
-exec handler.
-    |
-    v
-mid2...3
-    |
-    v
-mid1...3
-    |
-    v
-globalMiddleware...3
-```