Document functionality for creating notebooks programmatically

[takluyver]

I just noticed (while responding to jupyter/nbconvert#452) that the functions for building up notebooks (new_notebook(), new_code_cell(), etc.) are not documented.

[westurner]

• https://github.com/jupyter/nbformat/blob/master/nbformat/notebooknode.py
• https://github.com/jupyter/nbformat/blob/master/nbformat/v4/nbbase.py
• https://github.com/jupyter/nbformat/blob/master/nbformat/v4/tests/test_nbbase.py