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

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)