add map function to haskell section (#25718)

Add `map` function to haskell section of guide.freecodecamp.org

guide/english/haskell/map/index.md

@@ -0,0 +1,36 @@
+---
+title: map function
+---
+
+## `map` function
+
+The `map` function is a built-in haskell's function that maps a function to each element of a list and return a new list
+with the result. Graphically, it would look like this.
+```haskell
+map f [1,2,3,4]
+-- [f 1, f 2, f 3, f 4]
+```
+
+## Signature of `map`
+
+The signature of the map function is as follows
+```haskell
+map :: (a -> b) -> [a] -> [b]
+```
+
+## Examples
+```haskell
+map succ [1..5]
+-- [2,3,4,5,6]
+
+let sample = [(1, 2), (3, 1), (4, 10)]
+map fst sample
+-- [1,3,4]
+
+map snd sample
+-- [2,1,10]
+
+import Data.Char (toLower) # to get the toLower
+map toLower "HELLO WORLD HASKELL RULEZ!"
+-- "hello world haskell rulez!"
+```