Update Sphinx favicon config

[Timo Cornelius Metzger]

Thanks for using Sphinx Favicon in your project! I just released version 1.0 of the extension, which brings one breaking change: to better conform with Python standards, we changed the module name to sphinx_favicon (instead of sphinx-favicon). This means you'll have to update the name in the extensions list of your conf.py file (https://github.com/QCoDeS/Qcodes/blob/master/docs/conf.py#L81) to use version 1.0. Otherwise, your existing configuration should continue to work!

[Jens Hedegaard Nielsen]

@tcmetzger Thanks will be fixed via #5046