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/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)