Markdown indented code blocks (#20473)

* Implement Markdown indented code blocks

Additional indentation of 4 spaces makes a block an "indented code block"
(monospaced text without syntax highlighting).
Also `::` RST syntax for code blocks is disabled.

So instead of
```rst
see::

  Some code
```

the code block should be written as
```markdown
see:

    Some code
```

* Migrate RST literal blocks :: to Markdown's ones

lib/pure/distros.nim

@@ -9,12 +9,12 @@
 
 ## This module implements the basics for Linux distribution ("distro")
 ## detection and the OS's native package manager. Its primary purpose is to
-## produce output for Nimble packages, like::
+## produce output for Nimble packages, like:
 ##
-##   To complete the installation, run:
+##     To complete the installation, run:
 ##
-##   sudo apt-get install libblas-dev
-##   sudo apt-get install libvoodoo
+##     sudo apt-get install libblas-dev
+##     sudo apt-get install libvoodoo
 ##
 ## The above output could be the result of a code snippet like:
 ##

lib/pure/htmlgen.nim

@@ -34,9 +34,9 @@
 ##   var nim = "Nim"
 ##   echo h1(a(href="https://nim-lang.org", nim))
 ##
-## Writes the string::
+## Writes the string:
 ##
-##   <h1><a href="https://nim-lang.org">Nim</a></h1>
+##     <h1><a href="https://nim-lang.org">Nim</a></h1>
 ##
 
 import

lib/pure/os.nim

@@ -2294,11 +2294,12 @@ iterator walkDir*(dir: string; relative = false, checkDir = false):
   ##
   ## **Example:**
   ##
-  ## This directory structure::
-  ##   dirA / dirB / fileB1.txt
-  ##        / dirC
-  ##        / fileA1.txt
-  ##        / fileA2.txt
+  ## This directory structure:
+  ##
+  ##     dirA / dirB / fileB1.txt
+  ##          / dirC
+  ##          / fileA1.txt
+  ##          / fileA2.txt
   ##
   ## and this code:
   runnableExamples("-r:off"):

lib/pure/pegs.nim

@@ -2058,9 +2058,9 @@ func parsePeg*(pattern: string, filename = "pattern", line = 1, col = 0): Peg =
 
 func peg*(pattern: string): Peg =
   ## constructs a Peg object from the `pattern`. The short name has been
-  ## chosen to encourage its use as a raw string modifier::
+  ## chosen to encourage its use as a raw string modifier:
   ##
-  ##   peg"{\ident} \s* '=' \s* {.*}"
+  ##     peg"{\ident} \s* '=' \s* {.*}"
   result = parsePeg(pattern, "pattern")
 
 func escapePeg*(s: string): string =

lib/pure/strformat.nim

@@ -171,9 +171,9 @@ For strings and numeric types the optional argument is a so-called
 
 # Standard format specifiers for strings, integers and floats
 
-The general form of a standard format specifier is::
+The general form of a standard format specifier is:
 
-  [[fill]align][sign][#][0][minimumwidth][.precision][type]
+    [[fill]align][sign][#][0][minimumwidth][.precision][type]
 
 The square brackets `[]` indicate an optional element.
 
@@ -423,9 +423,9 @@ proc formatInt(n: SomeNumber; radix: int; spec: StandardFormatSpecifier): string
 proc parseStandardFormatSpecifier*(s: string; start = 0;
                                    ignoreUnknownSuffix = false): StandardFormatSpecifier =
   ## An exported helper proc that parses the "standard format specifiers",
-  ## as specified by the grammar::
+  ## as specified by the grammar:
   ##
-  ##   [[fill]align][sign][#][0][minimumwidth][.precision][type]
+  ##     [[fill]align][sign][#][0][minimumwidth][.precision][type]
   ##
   ## This is only of interest if you want to write a custom `format` proc that
   ## should support the standard format specifiers. If `ignoreUnknownSuffix` is true,

lib/pure/xmltree.nim

@@ -778,8 +778,8 @@ macro `<>`*(x: untyped): untyped =
   ## .. code-block:: nim
   ##   <>a(href="http://nim-lang.org", newText("Nim rules."))
   ##
-  ## Produces an XML tree for::
+  ## Produces an XML tree for:
   ##
-  ##  <a href="http://nim-lang.org">Nim rules.</a>
+  ##     <a href="http://nim-lang.org">Nim rules.</a>
   ##
   result = xmlConstructor(x)