complete std prefixes for stdlib (#22887)

follow up https://github.com/nim-lang/Nim/pull/22851
follow up https://github.com/nim-lang/Nim/pull/22873

doc/astspec.txt

@@ -893,7 +893,7 @@ on what keywords are present. Let's start with the simplest form.
 Concrete syntax:
 
   ```nim
-  import math
+  import std/math
   ```
 
 AST:
@@ -907,7 +907,7 @@ With ``except``, we get ``nnkImportExceptStmt``.
 Concrete syntax:
 
   ```nim
-  import math except pow
+  import std/math except pow
   ```
 
 AST:
@@ -916,13 +916,13 @@ AST:
   nnkImportExceptStmt(nnkIdent("math"),nnkIdent("pow"))
   ```
 
-Note that ``import math as m`` does not use a different node; rather,
+Note that ``import std/math as m`` does not use a different node; rather,
 we use ``nnkImportStmt`` with ``as`` as an infix operator.
 
 Concrete syntax:
 
   ```nim
-  import strutils as su
+  import std/strutils as su
   ```
 
 AST:
@@ -945,7 +945,7 @@ If we use ``from ... import``, the result is different, too.
 Concrete syntax:
 
   ```nim
-  from math import pow
+  from std/math import pow
   ```
 
 AST:
@@ -954,7 +954,7 @@ AST:
   nnkFromStmt(nnkIdent("math"), nnkIdent("pow"))
   ```
 
-Using ``from math as m import pow`` works identically to the ``as`` modifier
+Using ``from std/math as m import pow`` works identically to the ``as`` modifier
 with the ``import`` statement, but wrapped in ``nnkFromStmt``.
 
 Export statement

doc/docgen_sample.nim

@@ -1,6 +1,6 @@
 ## This module is a sample.
 
-import strutils
+import std/strutils
 
 proc helloWorld*(times: int) =
   ## Takes an integer and outputs

doc/estp.md

@@ -28,7 +28,7 @@ Otherwise your program is profiled.
 
 ```nim
 when compileOption("profiler"):
-  import nimprof
+  import std/nimprof
 ```
 
 After your program has finished the profiler will create a

doc/manual.md

@@ -655,7 +655,7 @@ string containing the literal. The callable identifier needs to be declared
 with a special ``'`` prefix:
 
   ```nim
-  import strutils
+  import std/strutils
   type u4 = distinct uint8 # a 4-bit unsigned integer aka "nibble"
   proc `'u4`(n: string): u4 =
     # The leading ' is required.
@@ -670,7 +670,7 @@ corresponds to this transformation. The transformation naturally handles
 the case that additional parameters are passed to the callee:
 
   ```nim
-  import strutils
+  import std/strutils
   type u4 = distinct uint8 # a 4-bit unsigned integer aka "nibble"
   proc `'u4`(n: string; moreData: int): u4 =
     result = (parseInt(n) and 0x0F).u4
@@ -5231,7 +5231,7 @@ conservative in its effect analysis:
   ```nim  test = "nim c $1"  status = 1
   {.push warningAsError[Effect]: on.}
 
-  import algorithm
+  import std/algorithm
 
   type
     MyInt = distinct int

doc/manual_experimental.md

@@ -1368,7 +1368,7 @@ to be computed dynamically.
   ```nim
   {.experimental: "dynamicBindSym".}
 
-  import macros
+  import std/macros
 
   macro callOp(opName, arg1, arg2): untyped =
     result = newCall(bindSym($opName), arg1, arg2)