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

tests/stdlib/trstgen.nim

@@ -632,7 +632,7 @@ Test literal block
 ::
 
   check """
-    let output1 = input1.toHtml
+    let output1 = input1.toHtml(preferRst)
     doAssert "<pre>" in output1
 
   test "Markdown code block":