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/packages/docutils/dochelpers.nim

@@ -75,10 +75,10 @@ proc toLangSymbol*(linkText: PRstNode): LangSymbol =
   ## Parses `linkText` into a more structured form using a state machine.
   ##
   ## This proc is designed to allow link syntax with operators even
-  ## without escaped backticks inside::
+  ## without escaped backticks inside:
   ##   
-  ##   `proc *`_
-  ##   `proc []`_
+  ##     `proc *`_
+  ##     `proc []`_
   ##
   ## This proc should be kept in sync with the `renderTypes` proc from
   ## ``compiler/typesrenderer.nim``.