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

doc/rstcommon.rst

@@ -1,9 +1,9 @@
 ..
   Usage of this file:
-     Add this in the beginning of *.rst file::
+     Add this in the beginning of *.rst file:
 
-       .. default-role:: code
-       .. include:: rstcommon.rst
+         .. default-role:: code
+         .. include:: rstcommon.rst
 
      It's the current trick for brevity and compatibility with both Github and
      rst2html.py, considering that Github cannot highlight Nim in