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/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"):