images on README won't get rendered on Pypi

[anilbey]

https://pypi.org/project/EModelRunner/

The solution is to update the URL to point to the raw version of the image.

E.g.

.. image:: docs/images/voltage-readme.png
.. image:: https://raw.githubusercontent.com/BlueBrain/BlueCelluLab/main/docs/images/voltage-readme.png