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/distros.nim

@@ -9,12 +9,12 @@
 
 ## This module implements the basics for Linux distribution ("distro")
 ## detection and the OS's native package manager. Its primary purpose is to
-## produce output for Nimble packages, like::
+## produce output for Nimble packages, like:
 ##
-##   To complete the installation, run:
+##     To complete the installation, run:
 ##
-##   sudo apt-get install libblas-dev
-##   sudo apt-get install libvoodoo
+##     sudo apt-get install libblas-dev
+##     sudo apt-get install libvoodoo
 ##
 ## The above output could be the result of a code snippet like:
 ##