* 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
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