docs v2: export png api

[manzt]

From previous docs

Saving the view
---------------

The currently visible HiGlass view can be downloaded to a file:

.. code-block:: python

  display.save_as_png('/tmp/my_view.png')

Not that this function can only be used within a Jupyter notebook
and works asynchronously so the saved screenshot will not nessarily
be complete immediately after the function finishes executing

[manzt]

I'm not sure this actually worked in the prior API? It would be something we could add to higlass-widget (not hg directly).

[manzt]

This feature was removed from higlass-python but the docs weren't updated. Would be nice to have, but we can track the issue with #57

[manzt]

It seems https://pypi.org/project/ipython-blocking/ might be relevant in implementing this.