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/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