fix(guide): simplify directory structure

guide/english/csharp/substring/index.md

@@ -0,0 +1,26 @@
+---
+title: Substring
+---
+
+# Substring
+
+`Substring` extracts a portion of a string value. It is used with 2 integer parameters, the first is location of the first character(starts with index 0) and the second is the desired character length.
+
+## Example
+```
+string firstSentence = "Apple, I have.";
+string secondSentence = "I have a Pen.";
+
+string apple = firstSentence.Substring(0,5);
+string pen = secondSentence.Substring(9,3);
+
+Console.WriteLine(apple);
+Console.WriteLine(pen);
+
+```
+
+## Output:
+```
+>Apple
+>Pen
+```