add support for note node type as blockquote markdown

[mmcky]

fixes #80

• add tests
• update documentation

[mmcky]

this PR is impacting on code_blocks rendering.

• review code_blocks test

This is currently failing as the code_blocks tests for code blocks that are contained in notes

.. NOTE::

   A code block in a note::

       >>> 1+1

   Another one::

       >>> 1+1

the current reference IPYNB actually ignores the note. Current implementation in this PR includes the contents of the note as raw text which is problematic when it contains code.

[mmcky]

I will merge this PR with the known limitation that note can currently only contain raw and direct markdown convertible text and open an issue to support embedded directives.