Docs: updates Jupyter notebook info

[tritemio]

The homepage needs to link more recent Jupyter Notebook docs and the reference beginner section in the reference docs need to be trimmed adding a link to:

http://jupyter-notebook-beginner-guide.readthedocs.org/

[tritemio]

Section updated: http://fretbursts.readthedocs.org/en/latest/absolute_beginner.html