Change stdlib imports to use std prefix in most examples (#17202)

doc/hcr.rst

@@ -29,7 +29,7 @@ To install SDL2 you can use ``nimble install sdl2``.
   import sdl2
 
   #*** import the hotcodereloading stdlib module ***
-  import hotcodereloading
+  import std/hotcodereloading
 
   var runGame*: bool = true
   var window: WindowPtr

doc/manual.rst

@@ -5941,7 +5941,7 @@ imported:
     :test: "nim c $1"
     :status: 1
 
-  import strutils except `%`, toUpperAscii
+  import std/strutils except `%`, toUpperAscii
 
   # doesn't work then:
   echo "$1" % "abc".toUpperAscii
@@ -5984,7 +5984,7 @@ Module names in imports
 A module alias can be introduced via the ``as`` keyword:
 
 .. code-block:: nim
-  import strutils as su, sequtils as qu
+  import std/strutils as su, std/sequtils as qu
 
   echo su.format("$1", "lalelu")
 
@@ -6048,7 +6048,7 @@ full qualification:
 .. code-block:: nim
     :test: "nim c $1"
 
-  from strutils import `%`
+  from std/strutils import `%`
 
   echo "$1" % "abc"
   # always possible: full qualification:

doc/spawn.txt

@@ -29,7 +29,7 @@ the passed expression on the thread pool and returns a `data flow variable`:idx:
 variables at the same time:
 
 .. code-block:: nim
-  import threadpool, ...
+  import std/threadpool, ...
 
   # wait until 2 out of 3 servers received the update:
   proc main =
@@ -56,7 +56,7 @@ Example:
 
 .. code-block:: nim
   # Compute PI in an inefficient way
-  import strutils, math, threadpool
+  import std/[strutils, math, threadpool]
 
   proc term(k: float): float = 4 * math.pow(-1, k) / (2*k + 1)