promote std prefix in docs (#17128)

doc/tut2.rst

@@ -182,7 +182,7 @@ for any type:
 
 .. code-block:: nim
     :test: "nim c $1"
-  import strutils
+  import std/strutils
 
   echo "abc".len # is the same as echo len("abc")
   echo "abc".toUpperAscii()
@@ -196,7 +196,7 @@ So "pure object oriented" code is easy to write:
 
 .. code-block:: nim
     :test: "nim c $1"
-  import strutils, sequtils
+  import std/[strutils, sequtils]
 
   stdout.writeLine("Give a list of numbers (separated by spaces): ")
   stdout.write(stdin.readLine.splitWhitespace.map(parseInt).max.`$`)
@@ -375,7 +375,7 @@ The ``try`` statement handles exceptions:
 
 .. code-block:: nim
     :test: "nim c $1"
-  from strutils import parseInt
+  from std/strutils import parseInt
 
   # read the first two lines of a text file that should contain numbers
   # and tries to add them
@@ -649,7 +649,7 @@ Example: Lifting Procs
 
 .. code-block:: nim
     :test: "nim c $1"
-  import math
+  import std/math
 
   template liftScalarProc(fname) =
     ## Lift a proc taking one scalar parameter and returning a